Update migration.md

Author: forstisabella

src/connections/sources/catalog/libraries/mobile/kotlin-android/migration.md

@@ -385,6 +385,9 @@ If you don't need to transform all of your Segment calls, and only want to trans
 
 ## 4. Upgrade Notes
 
+> info "You might need to call Identify as a one-off after migrating to Kotlin"
+> To preserve the userId for users identified prior to your migration to Kotlin, you must make a one-off Identify call. This is due to a storage format change between the Analytics for Android and Analytics-Kotlin libraries.
+
 ### 4.a) Changes to the Configuration Object
 
 The following option was renamed in Analytics-Kotlin: