diff --git a/java/src/com/android/inputmethod/latin/BinaryDictionary.java b/java/src/com/android/inputmethod/latin/BinaryDictionary.java
index 83ee982..94a1e36 100644
--- a/java/src/com/android/inputmethod/latin/BinaryDictionary.java
+++ b/java/src/com/android/inputmethod/latin/BinaryDictionary.java
@@ -373,8 +373,7 @@
         return getProbabilityNative(mNativeDict, codePoints);
     }
 
-    // TODO: Add a batch process version (isValidBigramMultiple?) to avoid excessive numbers of jni
-    // calls when checking for changes in an entire dictionary.
+    @UsedForTesting
     public boolean isValidBigram(final String word0, final String word1) {
         return getBigramProbability(word0, word1) != NOT_A_PROBABILITY;
     }
@@ -541,7 +540,9 @@
         close();
         final File dictFile = new File(mDictFilePath);
         final File tmpDictFile = new File(tmpDictFilePath);
-        FileUtils.deleteRecursively(dictFile);
+        if (!FileUtils.deleteRecursively(dictFile)) {
+            return false;
+        }
         if (!BinaryDictionaryUtils.renameDict(tmpDictFile, dictFile)) {
             return false;
         }
diff --git a/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java b/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
index b419817..6818c15 100644
--- a/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
+++ b/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
@@ -112,8 +112,9 @@
     }
 
     private boolean needsToMigrateDictionary(final int formatVersion) {
-        // TODO: Check version.
-        return false;
+        // When we bump up the dictionary format version, the old version should be added to here
+        // for supporting migration. Note that native code has to support reading such formats.
+        return formatVersion == FormatSpec.VERSION4_ONLY_FOR_TESTING;
     }
 
     public boolean isValidDictionaryLocked() {
diff --git a/java/src/com/android/inputmethod/latin/makedict/FormatSpec.java b/java/src/com/android/inputmethod/latin/makedict/FormatSpec.java
index f255034..613ff2b 100644
--- a/java/src/com/android/inputmethod/latin/makedict/FormatSpec.java
+++ b/java/src/com/android/inputmethod/latin/makedict/FormatSpec.java
@@ -186,7 +186,12 @@
     // From version 4 on, we use version * 100 + revision as a version number. That allows
     // us to change the format during development while having testing devices remove
     // older files with each upgrade, while still having a readable versioning scheme.
+    // When we bump up the dictionary format version, we should update
+    // ExpandableDictionary.needsToMigrateDictionary() and
+    // ExpandableDictionary.matchesExpectedBinaryDictFormatVersionForThisType().
     public static final int VERSION2 = 2;
+    // Dictionary version used for testing.
+    public static final int VERSION4_ONLY_FOR_TESTING = 399;
     public static final int VERSION4 = 401;
     static final int MINIMUM_SUPPORTED_VERSION = VERSION2;
     static final int MAXIMUM_SUPPORTED_VERSION = VERSION4;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
index 04e768f..4975512 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
@@ -50,12 +50,18 @@
 
 bool Ver4BigramListPolicy::addNewEntry(const int terminalId, const int newTargetTerminalId,
         const int newProbability, const int timestamp, bool *const outAddedNewEntry) {
+    // 1. The word has no bigrams yet.
+    // 2. The word has bigrams, and there is the target in the list.
+    // 3. The word has bigrams, and there is an invalid entry that can be reclaimed.
+    // 4. The word has bigrams. We have to append new bigram entry to the list.
+    // 5. Same as 4, but the list is the last entry of the content file.
+
     if (outAddedNewEntry) {
         *outAddedNewEntry = false;
     }
     const int bigramListPos = mBigramDictContent->getBigramListHeadPos(terminalId);
     if (bigramListPos == NOT_A_DICT_POS) {
-        // Updating PtNode that doesn't have a bigram list.
+        // Case 1. PtNode that doesn't have a bigram list.
         // Create new bigram list.
         if (!mBigramDictContent->createNewBigramList(terminalId)) {
             return false;
@@ -75,50 +81,55 @@
         return true;
     }
 
-    const int entryPosToUpdate = getEntryPosToUpdate(newTargetTerminalId, bigramListPos);
-    if (entryPosToUpdate != NOT_A_DICT_POS) {
-        // Overwrite existing entry.
-        const BigramEntry originalBigramEntry =
-                mBigramDictContent->getBigramEntry(entryPosToUpdate);
-        if (!originalBigramEntry.isValid()) {
-            // Reuse invalid entry.
-            if (outAddedNewEntry) {
-                *outAddedNewEntry = true;
+    int tailEntryPos = NOT_A_DICT_POS;
+    const int entryPosToUpdate = getEntryPosToUpdate(newTargetTerminalId, bigramListPos,
+            &tailEntryPos);
+    if (tailEntryPos != NOT_A_DICT_POS || entryPosToUpdate == NOT_A_DICT_POS) {
+        // Case 4, 5.
+        // Add new entry to the bigram list.
+        if (tailEntryPos == NOT_A_DICT_POS) {
+            // Case 4. Create new bigram list.
+            if (!mBigramDictContent->createNewBigramList(terminalId)) {
+                return false;
+            }
+            const int destPos = mBigramDictContent->getBigramListHeadPos(terminalId);
+            // Copy existing bigram list.
+            if (!mBigramDictContent->copyBigramList(bigramListPos, destPos, &tailEntryPos)) {
+                return false;
             }
         }
-        const BigramEntry updatedBigramEntry =
-                originalBigramEntry.updateTargetTerminalIdAndGetEntry(newTargetTerminalId);
+        // Write new entry at the tail position of the bigram content.
+        const BigramEntry newBigramEntry(false /* hasNext */, NOT_A_PROBABILITY,
+                newTargetTerminalId);
         const BigramEntry bigramEntryToWrite = createUpdatedBigramEntryFrom(
-                &updatedBigramEntry, newProbability, timestamp);
-        return mBigramDictContent->writeBigramEntry(&bigramEntryToWrite, entryPosToUpdate);
+                &newBigramEntry, newProbability, timestamp);
+        if (!mBigramDictContent->writeBigramEntryAtTail(&bigramEntryToWrite)) {
+            return false;
+        }
+        // Update has next flag of the tail entry.
+        if (!updateHasNextFlag(true /* hasNext */, tailEntryPos)) {
+            return false;
+        }
+        if (outAddedNewEntry) {
+            *outAddedNewEntry = true;
+        }
+        return true;
     }
 
-    // Add new entry to the bigram list.
-    // Create new bigram list.
-    if (!mBigramDictContent->createNewBigramList(terminalId)) {
-        return false;
+    // Case 2. Overwrite the existing entry. Case 3. Reclaim and reuse the existing invalid entry.
+    const BigramEntry originalBigramEntry = mBigramDictContent->getBigramEntry(entryPosToUpdate);
+    if (!originalBigramEntry.isValid()) {
+        // Case 3. Reuse the existing invalid entry. outAddedNewEntry is false when an existing
+        // entry is updated.
+        if (outAddedNewEntry) {
+            *outAddedNewEntry = true;
+        }
     }
-    int writingPos = mBigramDictContent->getBigramListHeadPos(terminalId);
-    int tailEntryPos = NOT_A_DICT_POS;
-    // Copy existing bigram list.
-    if (!mBigramDictContent->copyBigramList(bigramListPos, writingPos, &tailEntryPos)) {
-        return false;
-    }
-    // Write new entry at the tail position of the bigram content.
-    const BigramEntry newBigramEntry(false /* hasNext */, NOT_A_PROBABILITY, newTargetTerminalId);
+    const BigramEntry updatedBigramEntry =
+            originalBigramEntry.updateTargetTerminalIdAndGetEntry(newTargetTerminalId);
     const BigramEntry bigramEntryToWrite = createUpdatedBigramEntryFrom(
-            &newBigramEntry, newProbability, timestamp);
-    if (!mBigramDictContent->writeBigramEntryAtTail(&bigramEntryToWrite)) {
-        return false;
-    }
-    // Update has next flag of the tail entry.
-    if (!updateHasNextFlag(true /* hasNext */, tailEntryPos)) {
-        return false;
-    }
-    if (outAddedNewEntry) {
-        *outAddedNewEntry = true;
-    }
-    return true;
+            &updatedBigramEntry, newProbability, timestamp);
+    return mBigramDictContent->writeBigramEntry(&bigramEntryToWrite, entryPosToUpdate);
 }
 
 bool Ver4BigramListPolicy::removeEntry(const int terminalId, const int targetTerminalId) {
@@ -127,7 +138,8 @@
         // Bigram list doesn't exist.
         return false;
     }
-    const int entryPosToUpdate = getEntryPosToUpdate(targetTerminalId, bigramListPos);
+    const int entryPosToUpdate = getEntryPosToUpdate(targetTerminalId, bigramListPos,
+            nullptr /* outTailEntryPos */);
     if (entryPosToUpdate == NOT_A_DICT_POS) {
         // Bigram entry doesn't exist.
         return false;
@@ -212,7 +224,10 @@
 }
 
 int Ver4BigramListPolicy::getEntryPosToUpdate(const int targetTerminalIdToFind,
-        const int bigramListPos) const {
+        const int bigramListPos, int *const outTailEntryPos) const {
+    if (outTailEntryPos) {
+        *outTailEntryPos = NOT_A_DICT_POS;
+    }
     bool hasNext = true;
     int invalidEntryPos = NOT_A_DICT_POS;
     int readingPos = bigramListPos;
@@ -228,6 +243,11 @@
             // Invalid entry that can be reused is found.
             invalidEntryPos = entryPos;
         }
+        if (!hasNext && mBigramDictContent->isContentTailPos(readingPos)) {
+            if (outTailEntryPos) {
+                *outTailEntryPos = entryPos;
+            }
+        }
     }
     return invalidEntryPos;
 }
diff --git a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h
index d8f7be6..c1f3335 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h
@@ -56,7 +56,8 @@
  private:
     DISALLOW_IMPLICIT_CONSTRUCTORS(Ver4BigramListPolicy);
 
-    int getEntryPosToUpdate(const int targetTerminalIdToFind, const int bigramListPos) const;
+    int getEntryPosToUpdate(const int targetTerminalIdToFind, const int bigramListPos,
+            int *const outTailEntryPos) const;
 
     const BigramEntry createUpdatedBigramEntryFrom(const BigramEntry *const originalBigramEntry,
             const int newProbability, const int timestamp) const;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
index e4a6dc5..da24302 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
@@ -139,6 +139,8 @@
         switch (mDictFormatVersion) {
             case FormatUtils::VERSION_2:
                 return FormatUtils::VERSION_2;
+            case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
+                return FormatUtils::VERSION_4_ONLY_FOR_TESTING;
             case FormatUtils::VERSION_4:
                 return FormatUtils::VERSION_4;
             default:
diff --git a/native/jni/src/suggest/policyimpl/dictionary/header/header_read_write_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/header/header_read_write_utils.cpp
index 5608e27..2a9028a 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/header/header_read_write_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/header/header_read_write_utils.cpp
@@ -98,8 +98,9 @@
         case FormatUtils::VERSION_2:
             // Version 2 dictionary writing is not supported.
             return false;
+        case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
         case FormatUtils::VERSION_4:
-            return buffer->writeUintAndAdvancePosition(FormatUtils::VERSION_4 /* data */,
+            return buffer->writeUintAndAdvancePosition(version /* data */,
                     HEADER_DICTIONARY_VERSION_SIZE, writingPos);
         default:
             return false;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/dictionary_structure_with_buffer_policy_factory.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/dictionary_structure_with_buffer_policy_factory.cpp
index be7e43b..c4d1860 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/dictionary_structure_with_buffer_policy_factory.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/dictionary_structure_with_buffer_policy_factory.cpp
@@ -52,9 +52,11 @@
         DictionaryStructureWithBufferPolicyFactory:: newPolicyForOnMemoryDict(
                 const int formatVersion, const std::vector<int> &locale,
                 const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap) {
-    switch (formatVersion) {
+    FormatUtils::FORMAT_VERSION dictFormatVersion = FormatUtils::getFormatVersion(formatVersion);
+    switch (dictFormatVersion) {
+        case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
         case FormatUtils::VERSION_4: {
-            HeaderPolicy headerPolicy(FormatUtils::VERSION_4, locale, attributeMap);
+            HeaderPolicy headerPolicy(dictFormatVersion, locale, attributeMap);
             Ver4DictBuffers::Ver4DictBuffersPtr dictBuffers =
                     Ver4DictBuffers::createVer4DictBuffers(&headerPolicy,
                             Ver4DictConstants::MAX_DICT_EXTENDED_REGION_SIZE);
@@ -87,11 +89,13 @@
     if (!mmappedBuffer) {
         return DictionaryStructureWithBufferPolicy::StructurePolicyPtr(nullptr);
     }
-    switch (FormatUtils::detectFormatVersion(mmappedBuffer->getBuffer(),
-            mmappedBuffer->getBufferSize())) {
+    const FormatUtils::FORMAT_VERSION formatVersion = FormatUtils::detectFormatVersion(
+            mmappedBuffer->getBuffer(), mmappedBuffer->getBufferSize());
+    switch (formatVersion) {
         case FormatUtils::VERSION_2:
             AKLOGE("Given path is a directory but the format is version 2. path: %s", path);
             break;
+        case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
         case FormatUtils::VERSION_4: {
             const int dictDirPathBufSize = strlen(headerFilePath) + 1 /* terminator */;
             char dictPath[dictDirPathBufSize];
@@ -102,7 +106,8 @@
                 return DictionaryStructureWithBufferPolicy::StructurePolicyPtr(nullptr);
             }
             Ver4DictBuffers::Ver4DictBuffersPtr dictBuffers(
-                    Ver4DictBuffers::openVer4DictBuffers(dictPath, std::move(mmappedBuffer)));
+                    Ver4DictBuffers::openVer4DictBuffers(dictPath, std::move(mmappedBuffer),
+                            formatVersion));
             if (!dictBuffers || !dictBuffers->isValid()) {
                 AKLOGE("DICT: The dictionary doesn't satisfy ver4 format requirements. path: %s",
                         path);
@@ -135,6 +140,7 @@
         case FormatUtils::VERSION_2:
             return DictionaryStructureWithBufferPolicy::StructurePolicyPtr(
                     new PatriciaTriePolicy(std::move(mmappedBuffer)));
+        case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
         case FormatUtils::VERSION_4:
             AKLOGE("Given path is a file but the format is version 4. path: %s", path);
             break;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/content/bigram_dict_content.h b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/content/bigram_dict_content.h
index 40ece76..944e0f9 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/content/bigram_dict_content.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/content/bigram_dict_content.h
@@ -88,6 +88,10 @@
             const BigramDictContent *const originalBigramDictContent,
             int *const outBigramEntryCount);
 
+    bool isContentTailPos(const int pos) const {
+        return pos == getContentBuffer()->getTailPosition();
+    }
+
  private:
     DISALLOW_COPY_AND_ASSIGN(BigramDictContent);
 
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.cpp
index 95f6544..77ed38b 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.cpp
@@ -27,7 +27,8 @@
 namespace latinime {
 
 /* static */ Ver4DictBuffers::Ver4DictBuffersPtr Ver4DictBuffers::openVer4DictBuffers(
-        const char *const dictPath, MmappedBuffer::MmappedBufferPtr headerBuffer) {
+        const char *const dictPath, MmappedBuffer::MmappedBufferPtr headerBuffer,
+        const FormatUtils::FORMAT_VERSION formatVersion) {
     if (!headerBuffer) {
         ASSERT(false);
         AKLOGE("The header buffer must be valid to open ver4 dict buffers.");
@@ -35,7 +36,8 @@
     }
     // TODO: take only dictDirPath, and open both header and trie files in the constructor below
     const bool isUpdatable = headerBuffer->isUpdatable();
-    return Ver4DictBuffersPtr(new Ver4DictBuffers(dictPath, std::move(headerBuffer), isUpdatable));
+    return Ver4DictBuffersPtr(new Ver4DictBuffers(dictPath, std::move(headerBuffer), isUpdatable,
+            formatVersion));
 }
 
 bool Ver4DictBuffers::flushHeaderAndDictBuffers(const char *const dictDirPath,
@@ -113,11 +115,12 @@
 }
 
 Ver4DictBuffers::Ver4DictBuffers(const char *const dictPath,
-        MmappedBuffer::MmappedBufferPtr headerBuffer, const bool isUpdatable)
+        MmappedBuffer::MmappedBufferPtr headerBuffer, const bool isUpdatable,
+        const FormatUtils::FORMAT_VERSION formatVersion)
         : mHeaderBuffer(std::move(headerBuffer)),
           mDictBuffer(MmappedBuffer::openBuffer(dictPath,
                   Ver4DictConstants::TRIE_FILE_EXTENSION, isUpdatable)),
-          mHeaderPolicy(mHeaderBuffer->getBuffer(), FormatUtils::VERSION_4),
+          mHeaderPolicy(mHeaderBuffer->getBuffer(), formatVersion),
           mExpandableHeaderBuffer(mHeaderBuffer ? mHeaderBuffer->getBuffer() : nullptr,
                   mHeaderPolicy.getSize(),
                   BufferWithExtendableBuffer::DEFAULT_MAX_ADDITIONAL_BUFFER_SIZE),
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h
index fc41432..df177c1 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h
@@ -36,7 +36,8 @@
     typedef std::unique_ptr<Ver4DictBuffers> Ver4DictBuffersPtr;
 
     static Ver4DictBuffersPtr openVer4DictBuffers(const char *const dictDirPath,
-            MmappedBuffer::MmappedBufferPtr headerBuffer);
+            MmappedBuffer::MmappedBufferPtr headerBuffer,
+            const FormatUtils::FORMAT_VERSION formatVersion);
 
     static AK_FORCE_INLINE Ver4DictBuffersPtr createVer4DictBuffers(
             const HeaderPolicy *const headerPolicy, const int maxTrieSize) {
@@ -120,7 +121,8 @@
     DISALLOW_COPY_AND_ASSIGN(Ver4DictBuffers);
 
     Ver4DictBuffers(const char *const dictDirPath,
-            const MmappedBuffer::MmappedBufferPtr headerBuffer, const bool isUpdatable);
+            const MmappedBuffer::MmappedBufferPtr headerBuffer, const bool isUpdatable,
+            const FormatUtils::FORMAT_VERSION formatVersion);
 
     Ver4DictBuffers(const HeaderPolicy *const headerPolicy, const int maxTrieSize);
 
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.cpp
index 87fa599..7bc7b0a 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.cpp
@@ -34,9 +34,12 @@
         const int dictVersion, const std::vector<int> localeAsCodePointVector,
         const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap) {
     TimeKeeper::setCurrentTime();
-    switch (dictVersion) {
+    const FormatUtils::FORMAT_VERSION formatVersion = FormatUtils::getFormatVersion(dictVersion);
+    switch (formatVersion) {
+        case FormatUtils::VERSION_4_ONLY_FOR_TESTING:
         case FormatUtils::VERSION_4:
-            return createEmptyV4DictFile(filePath, localeAsCodePointVector, attributeMap);
+            return createEmptyV4DictFile(filePath, localeAsCodePointVector, attributeMap,
+                    formatVersion);
         default:
             AKLOGE("Cannot create dictionary %s because format version %d is not supported.",
                     filePath, dictVersion);
@@ -46,8 +49,9 @@
 
 /* static */ bool DictFileWritingUtils::createEmptyV4DictFile(const char *const dirPath,
         const std::vector<int> localeAsCodePointVector,
-        const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap) {
-    HeaderPolicy headerPolicy(FormatUtils::VERSION_4, localeAsCodePointVector, attributeMap);
+        const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap,
+        const FormatUtils::FORMAT_VERSION formatVersion) {
+    HeaderPolicy headerPolicy(formatVersion, localeAsCodePointVector, attributeMap);
     Ver4DictBuffers::Ver4DictBuffersPtr dictBuffers(
             Ver4DictBuffers::createVer4DictBuffers(&headerPolicy,
                     Ver4DictConstants::MAX_DICT_EXTENDED_REGION_SIZE));
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.h b/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.h
index 54ec651..a822989 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/dict_file_writing_utils.h
@@ -21,6 +21,7 @@
 
 #include "defines.h"
 #include "suggest/policyimpl/dictionary/header/header_read_write_utils.h"
+#include "suggest/policyimpl/dictionary/utils/format_utils.h"
 
 namespace latinime {
 
@@ -46,7 +47,8 @@
 
     static bool createEmptyV4DictFile(const char *const filePath,
             const std::vector<int> localeAsCodePointVector,
-            const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap);
+            const DictionaryHeaderStructurePolicy::AttributeMap *const attributeMap,
+            const FormatUtils::FORMAT_VERSION formatVersion);
 
     static bool flushBufferToFile(const char *const filePath,
             const BufferWithExtendableBuffer *const buffer);
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.cpp
index cd3c403..a8518cd 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.cpp
@@ -25,6 +25,18 @@
 // Magic number (4 bytes), version (2 bytes), flags (2 bytes), header size (4 bytes) = 12
 const int FormatUtils::DICTIONARY_MINIMUM_SIZE = 12;
 
+/* static */ FormatUtils::FORMAT_VERSION FormatUtils::getFormatVersion(const int formatVersion) {
+    switch (formatVersion) {
+        case VERSION_2:
+            return VERSION_2;
+        case VERSION_4_ONLY_FOR_TESTING:
+            return VERSION_4_ONLY_FOR_TESTING;
+        case VERSION_4:
+            return VERSION_4;
+        default:
+            return UNKNOWN_VERSION;
+    }
+}
 /* static */ FormatUtils::FORMAT_VERSION FormatUtils::detectFormatVersion(
         const uint8_t *const dict, const int dictSize) {
     // The magic number is stored big-endian.
@@ -46,6 +58,8 @@
             // same so we use them for both here.
             if (ByteArrayUtils::readUint16(dict, 4) == VERSION_2) {
                 return VERSION_2;
+            } else if (ByteArrayUtils::readUint16(dict, 4) == VERSION_4_ONLY_FOR_TESTING) {
+                return VERSION_4_ONLY_FOR_TESTING;
             } else if (ByteArrayUtils::readUint16(dict, 4) == VERSION_4) {
                 return VERSION_4;
             } else {
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.h b/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.h
index 759b1c9..20dfb9d 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/format_utils.h
@@ -31,6 +31,7 @@
     enum FORMAT_VERSION {
         // These MUST have the same values as the relevant constants in FormatSpec.java.
         VERSION_2 = 2,
+        VERSION_4_ONLY_FOR_TESTING = 399,
         VERSION_4 = 401,
         UNKNOWN_VERSION = -1
     };
@@ -39,6 +40,7 @@
     // unsupported or obsolete dictionary formats.
     static const uint32_t MAGIC_NUMBER;
 
+    static FORMAT_VERSION getFormatVersion(const int formatVersion);
     static FORMAT_VERSION detectFormatVersion(const uint8_t *const dict, const int dictSize);
 
  private:
diff --git a/tests/src/com/android/inputmethod/latin/BinaryDictionaryTests.java b/tests/src/com/android/inputmethod/latin/BinaryDictionaryTests.java
index 0fb0fa5..9ceafa7 100644
--- a/tests/src/com/android/inputmethod/latin/BinaryDictionaryTests.java
+++ b/tests/src/com/android/inputmethod/latin/BinaryDictionaryTests.java
@@ -46,21 +46,23 @@
 
     private File createEmptyDictionaryAndGetFile(final String dictId,
             final int formatVersion) throws IOException {
-       if (formatVersion == FormatSpec.VERSION4) {
-            return createEmptyVer4DictionaryAndGetFile(dictId);
+        if (formatVersion == FormatSpec.VERSION4
+                || formatVersion == FormatSpec.VERSION4_ONLY_FOR_TESTING) {
+            return createEmptyVer4DictionaryAndGetFile(dictId, formatVersion);
         } else {
             throw new IOException("Dictionary format version " + formatVersion
                     + " is not supported.");
         }
     }
 
-    private File createEmptyVer4DictionaryAndGetFile(final String dictId) throws IOException {
+    private File createEmptyVer4DictionaryAndGetFile(final String dictId,
+            final int formatVersion) throws IOException {
         final File file = File.createTempFile(dictId, TEST_DICT_FILE_EXTENSION,
                 getContext().getCacheDir());
         file.delete();
         file.mkdir();
         Map<String, String> attributeMap = new HashMap<String, String>();
-        if (BinaryDictionaryUtils.createEmptyDictFile(file.getAbsolutePath(), FormatSpec.VERSION4,
+        if (BinaryDictionaryUtils.createEmptyDictFile(file.getAbsolutePath(), formatVersion,
                 Locale.ENGLISH, attributeMap)) {
             return file;
         } else {
@@ -1223,4 +1225,36 @@
             }
         }
     }
+
+    public void testDictMigration() {
+        testDictMigration(FormatSpec.VERSION4_ONLY_FOR_TESTING, FormatSpec.VERSION4);
+    }
+
+    private void testDictMigration(final int fromFormatVersion, final int toFormatVersion) {
+        File dictFile = null;
+        try {
+            dictFile = createEmptyDictionaryAndGetFile("TestBinaryDictionary", fromFormatVersion);
+        } catch (IOException e) {
+            fail("IOException while writing an initial dictionary : " + e);
+        }
+        final BinaryDictionary binaryDictionary = new BinaryDictionary(dictFile.getAbsolutePath(),
+                0 /* offset */, dictFile.length(), true /* useFullEditDistance */,
+                Locale.getDefault(), TEST_LOCALE, true /* isUpdatable */);
+        final int unigramProbability = 100;
+        addUnigramWord(binaryDictionary, "aaa", unigramProbability);
+        addUnigramWord(binaryDictionary, "bbb", unigramProbability);
+        final int bigramProbability = 10;
+        addBigramWords(binaryDictionary, "aaa", "bbb", bigramProbability);
+        assertEquals(unigramProbability, binaryDictionary.getFrequency("aaa"));
+        assertEquals(unigramProbability, binaryDictionary.getFrequency("bbb"));
+        assertTrue(binaryDictionary.isValidBigram("aaa", "bbb"));
+        assertEquals(fromFormatVersion, binaryDictionary.getFormatVersion());
+        assertTrue(binaryDictionary.migrateTo(toFormatVersion));
+        assertTrue(binaryDictionary.isValidDictionary());
+        assertEquals(toFormatVersion, binaryDictionary.getFormatVersion());
+        assertEquals(unigramProbability, binaryDictionary.getFrequency("aaa"));
+        assertEquals(unigramProbability, binaryDictionary.getFrequency("bbb"));
+        // TODO: Add tests for bigram frequency when the implementation gets ready.
+        assertTrue(binaryDictionary.isValidBigram("aaa", "bbb"));
+    }
 }