diff --git a/native/jni/com_android_inputmethod_latin_BinaryDictionary.cpp b/native/jni/com_android_inputmethod_latin_BinaryDictionary.cpp
index 3ac424f..a3d8ec1 100644
--- a/native/jni/com_android_inputmethod_latin_BinaryDictionary.cpp
+++ b/native/jni/com_android_inputmethod_latin_BinaryDictionary.cpp
@@ -335,8 +335,9 @@
     if (!shortcutTargetCodePoints.empty()) {
         shortcuts.emplace_back(&shortcutTargetCodePoints, shortcutProbability);
     }
+    // Use 1 for count to indicate the word has inputed.
     const UnigramProperty unigramProperty(isNotAWord, isBlacklisted,
-            probability, timestamp, 0 /* level */, 0 /* count */, &shortcuts);
+            probability, timestamp, 0 /* level */, 1 /* count */, &shortcuts);
     dictionary->addUnigramWord(codePoints, codePointCount, &unigramProperty);
 }
 
@@ -436,8 +437,9 @@
                     env->GetIntField(languageModelParam, shortcutProbabilityFieldId);
             shortcuts.emplace_back(&shortcutTargetCodePoints, shortcutProbability);
         }
+        // Use 1 for count to indicate the word has inputed.
         const UnigramProperty unigramProperty(isNotAWord, isBlacklisted,
-                unigramProbability, timestamp, 0 /* level */, 0 /* count */, &shortcuts);
+                unigramProbability, timestamp, 0 /* level */, 1 /* count */, &shortcuts);
         dictionary->addUnigramWord(word1CodePoints, word1Length, &unigramProperty);
         if (word0) {
             jint bigramProbability = env->GetIntField(languageModelParam, bigramProbabilityFieldId);
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 4975512..1645039 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
@@ -257,10 +257,12 @@
         const int timestamp) const {
     // TODO: Consolidate historical info and probability.
     if (mHeaderPolicy->hasHistoricalInfoOfWords()) {
+        // Use 1 for count to indicate the bigram has inputed.
+        const HistoricalInfo historicalInfoForUpdate(timestamp, 0 /* level */, 1 /* count */);
         const HistoricalInfo updatedHistoricalInfo =
                 ForgettingCurveUtils::createUpdatedHistoricalInfo(
-                        originalBigramEntry->getHistoricalInfo(), newProbability, timestamp,
-                        mHeaderPolicy);
+                        originalBigramEntry->getHistoricalInfo(), newProbability,
+                        &historicalInfoForUpdate, mHeaderPolicy);
         return originalBigramEntry->updateHistoricalInfoAndGetEntry(&updatedHistoricalInfo);
     } else {
         return originalBigramEntry->updateProbabilityAndGetEntry(newProbability);
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
index 50a3e56..cc3a24a 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
@@ -387,11 +387,12 @@
         const UnigramProperty *const unigramProperty) const {
     // TODO: Consolidate historical info and probability.
     if (mHeaderPolicy->hasHistoricalInfoOfWords()) {
+        const HistoricalInfo historicalInfoForUpdate(unigramProperty->getTimestamp(),
+                unigramProperty->getLevel(), unigramProperty->getCount());
         const HistoricalInfo updatedHistoricalInfo =
                 ForgettingCurveUtils::createUpdatedHistoricalInfo(
                         originalProbabilityEntry->getHistoricalInfo(),
-                        unigramProperty->getProbability(), unigramProperty->getTimestamp(),
-                        mHeaderPolicy);
+                        unigramProperty->getProbability(), &historicalInfoForUpdate, mHeaderPolicy);
         return originalProbabilityEntry->createEntryWithUpdatedHistoricalInfo(
                 &updatedHistoricalInfo);
     } else {
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp
index 2584fe5..9999e06 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp
@@ -425,6 +425,9 @@
 }
 
 int Ver4PatriciaTriePolicy::getNextWordAndNextToken(const int token, int *const outCodePoints) {
+    // TODO: Return code point count like other methods.
+    // Null termination.
+    outCodePoints[0] = 0;
     if (token == 0) {
         mTerminalPtNodePositionsForIteratingWords.clear();
         DynamicPtReadingHelper::TraversePolicyToGetAllTerminalPtNodePositions traversePolicy(
@@ -441,8 +444,13 @@
     }
     const int terminalPtNodePos = mTerminalPtNodePositionsForIteratingWords[token];
     int unigramProbability = NOT_A_PROBABILITY;
-    getCodePointsAndProbabilityAndReturnCodePointCount(terminalPtNodePos, MAX_WORD_LENGTH,
-            outCodePoints, &unigramProbability);
+    const int codePointCount = getCodePointsAndProbabilityAndReturnCodePointCount(
+            terminalPtNodePos, MAX_WORD_LENGTH, outCodePoints, &unigramProbability);
+    if (codePointCount < MAX_WORD_LENGTH) {
+        // Null termination. outCodePoints have to be null terminated or contain MAX_WORD_LENGTH
+        // code points.
+        outCodePoints[codePointCount] = 0;
+    }
     const int nextToken = token + 1;
     if (nextToken >= terminalPtNodePositionsVectorSize) {
         // All words have been iterated.
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
index c7d3df9..fed0ae7 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
@@ -30,7 +30,7 @@
 const int ForgettingCurveUtils::DECAY_INTERVAL_SECONDS = 2 * 60 * 60;
 
 const int ForgettingCurveUtils::MAX_LEVEL = 3;
-const int ForgettingCurveUtils::MIN_VALID_LEVEL = 1;
+const int ForgettingCurveUtils::MIN_VISIBLE_LEVEL = 1;
 const int ForgettingCurveUtils::MAX_ELAPSED_TIME_STEP_COUNT = 15;
 const int ForgettingCurveUtils::DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD = 14;
 
@@ -41,25 +41,34 @@
 
 // TODO: Revise the logic to decide the initial probability depending on the given probability.
 /* static */ const HistoricalInfo ForgettingCurveUtils::createUpdatedHistoricalInfo(
-        const HistoricalInfo *const originalHistoricalInfo,
-        const int newProbability, const int timestamp, const HeaderPolicy *const headerPolicy) {
+        const HistoricalInfo *const originalHistoricalInfo, const int newProbability,
+        const HistoricalInfo *const newHistoricalInfo, const HeaderPolicy *const headerPolicy) {
+    const int timestamp = newHistoricalInfo->getTimeStamp();
     if (newProbability != NOT_A_PROBABILITY && originalHistoricalInfo->getLevel() == 0) {
-        return HistoricalInfo(timestamp, MIN_VALID_LEVEL /* level */, 0 /* count */);
-    } else if (!originalHistoricalInfo->isValid()) {
+        // Add entry as a valid word.
+        const int level = clampToVisibleEntryLevelRange(newHistoricalInfo->getLevel());
+        const int count = clampToValidCountRange(newHistoricalInfo->getCount(), headerPolicy);
+        return HistoricalInfo(timestamp, level, count);
+    } else if (!originalHistoricalInfo->isValid()
+            || originalHistoricalInfo->getLevel() < newHistoricalInfo->getLevel()
+            || (originalHistoricalInfo->getLevel() == newHistoricalInfo->getLevel()
+                    && originalHistoricalInfo->getCount() < newHistoricalInfo->getCount())) {
         // Initial information.
-        return HistoricalInfo(timestamp, 0 /* level */, 1 /* count */);
+        const int level = clampToValidLevelRange(newHistoricalInfo->getLevel());
+        const int count = clampToValidCountRange(newHistoricalInfo->getCount(), headerPolicy);
+        return HistoricalInfo(timestamp, level, count);
     } else {
         const int updatedCount = originalHistoricalInfo->getCount() + 1;
         if (updatedCount >= headerPolicy->getForgettingCurveOccurrencesToLevelUp()) {
             // The count exceeds the max value the level can be incremented.
             if (originalHistoricalInfo->getLevel() >= MAX_LEVEL) {
                 // The level is already max.
-                return HistoricalInfo(timestamp, originalHistoricalInfo->getLevel(),
-                        originalHistoricalInfo->getCount());
+                return HistoricalInfo(timestamp,
+                        originalHistoricalInfo->getLevel(), originalHistoricalInfo->getCount());
             } else {
                 // Level up.
-                return HistoricalInfo(timestamp, originalHistoricalInfo->getLevel() + 1,
-                        0 /* count */);
+                return HistoricalInfo(timestamp,
+                        originalHistoricalInfo->getLevel() + 1, 0 /* count */);
             }
         } else {
             return HistoricalInfo(timestamp, originalHistoricalInfo->getLevel(), updatedCount);
@@ -73,8 +82,8 @@
             headerPolicy->getForgettingCurveDurationToLevelDown());
     return sProbabilityTable.getProbability(
             headerPolicy->getForgettingCurveProbabilityValuesTableId(),
-            std::min(std::max(historicalInfo->getLevel(), 0), MAX_LEVEL),
-            std::min(std::max(elapsedTimeStepCount, 0), MAX_ELAPSED_TIME_STEP_COUNT));
+            clampToValidLevelRange(historicalInfo->getLevel()),
+            clampToValidTimeStepCountRange(elapsedTimeStepCount));
 }
 
 /* static */ int ForgettingCurveUtils::getProbability(const int unigramProbability,
@@ -155,6 +164,23 @@
     return elapsedTimeInSeconds / timeStepDurationInSeconds;
 }
 
+/* static */ int ForgettingCurveUtils::clampToVisibleEntryLevelRange(const int level) {
+    return std::min(std::max(level, MIN_VISIBLE_LEVEL), MAX_LEVEL);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidCountRange(const int count,
+        const HeaderPolicy *const headerPolicy) {
+    return std::min(std::max(count, 0), headerPolicy->getForgettingCurveOccurrencesToLevelUp() - 1);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidLevelRange(const int level) {
+    return std::min(std::max(level, 0), MAX_LEVEL);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidTimeStepCountRange(const int timeStepCount) {
+    return std::min(std::max(timeStepCount, 0), MAX_ELAPSED_TIME_STEP_COUNT);
+}
+
 const int ForgettingCurveUtils::ProbabilityTable::PROBABILITY_TABLE_COUNT = 4;
 const int ForgettingCurveUtils::ProbabilityTable::WEAK_PROBABILITY_TABLE_ID = 0;
 const int ForgettingCurveUtils::ProbabilityTable::MODEST_PROBABILITY_TABLE_ID = 1;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
index bb86909..3ff80ae 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
@@ -30,7 +30,7 @@
  public:
     static const HistoricalInfo createUpdatedHistoricalInfo(
             const HistoricalInfo *const originalHistoricalInfo, const int newProbability,
-            const int timestamp, const HeaderPolicy *const headerPolicy);
+            const HistoricalInfo *const newHistoricalInfo, const HeaderPolicy *const headerPolicy);
 
     static const HistoricalInfo createHistoricalInfoToSave(
             const HistoricalInfo *const originalHistoricalInfo,
@@ -93,7 +93,7 @@
     static const int DECAY_INTERVAL_SECONDS;
 
     static const int MAX_LEVEL;
-    static const int MIN_VALID_LEVEL;
+    static const int MIN_VISIBLE_LEVEL;
     static const int MAX_ELAPSED_TIME_STEP_COUNT;
     static const int DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
 
@@ -103,8 +103,11 @@
     static const ProbabilityTable sProbabilityTable;
 
     static int backoff(const int unigramProbability);
-
     static int getElapsedTimeStepCount(const int timestamp, const int durationToLevelDown);
+    static int clampToVisibleEntryLevelRange(const int level);
+    static int clampToValidLevelRange(const int level);
+    static int clampToValidCountRange(const int count, const HeaderPolicy *const headerPolicy);
+    static int clampToValidTimeStepCountRange(const int timeStepCount);
 };
 } // namespace latinime
 #endif /* LATINIME_FORGETTING_CURVE_UTILS_H */
diff --git a/tests/src/com/android/inputmethod/latin/BinaryDictionaryDecayingTests.java b/tests/src/com/android/inputmethod/latin/BinaryDictionaryDecayingTests.java
index ae2205b..aed24c5 100644
--- a/tests/src/com/android/inputmethod/latin/BinaryDictionaryDecayingTests.java
+++ b/tests/src/com/android/inputmethod/latin/BinaryDictionaryDecayingTests.java
@@ -93,15 +93,17 @@
 
     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());
         FileUtils.deleteRecursively(file);
@@ -113,7 +115,7 @@
                 DictionaryHeader.ATTRIBUTE_VALUE_TRUE);
         attributeMap.put(DictionaryHeader.HAS_HISTORICAL_INFO_KEY,
                 DictionaryHeader.ATTRIBUTE_VALUE_TRUE);
-        if (BinaryDictionaryUtils.createEmptyDictFile(file.getAbsolutePath(), FormatSpec.VERSION4,
+        if (BinaryDictionaryUtils.createEmptyDictFile(file.getAbsolutePath(), formatVersion,
                 LocaleUtils.constructLocaleFromString(TEST_LOCALE), attributeMap)) {
             return file;
         } else {
@@ -562,4 +564,43 @@
             }
         }
     }
+
+    public void testDictMigration() {
+        testDictMigration(FormatSpec.VERSION4_ONLY_FOR_TESTING, FormatSpec.VERSION4);
+    }
+
+    private void testDictMigration(final int fromFormatVersion, final int toFormatVersion) {
+        setCurrentTimeForTestMode(mCurrentTime);
+        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 */);
+        // TODO: Add tests for bigrams when the implementation gets ready.
+        addUnigramWord(binaryDictionary, "aaa", DUMMY_PROBABILITY);
+        assertTrue(binaryDictionary.isValidWord("aaa"));
+        addUnigramWord(binaryDictionary, "bbb", Dictionary.NOT_A_PROBABILITY);
+        assertFalse(binaryDictionary.isValidWord("bbb"));
+        addUnigramWord(binaryDictionary, "ccc", DUMMY_PROBABILITY);
+        addUnigramWord(binaryDictionary, "ccc", DUMMY_PROBABILITY);
+        addUnigramWord(binaryDictionary, "ccc", DUMMY_PROBABILITY);
+        addUnigramWord(binaryDictionary, "ccc", DUMMY_PROBABILITY);
+        addUnigramWord(binaryDictionary, "ccc", DUMMY_PROBABILITY);
+
+        assertEquals(fromFormatVersion, binaryDictionary.getFormatVersion());
+        assertTrue(binaryDictionary.migrateTo(toFormatVersion));
+        assertTrue(binaryDictionary.isValidDictionary());
+        assertEquals(toFormatVersion, binaryDictionary.getFormatVersion());
+        assertTrue(binaryDictionary.isValidWord("aaa"));
+        assertFalse(binaryDictionary.isValidWord("bbb"));
+        assertTrue(binaryDictionary.getFrequency("aaa") < binaryDictionary.getFrequency("ccc"));
+        addUnigramWord(binaryDictionary, "bbb", Dictionary.NOT_A_PROBABILITY);
+        assertTrue(binaryDictionary.isValidWord("bbb"));
+        binaryDictionary.close();
+        dictFile.delete();
+    }
 }