UIMA has some limited support for typed arrays. These are declared in type system descriptors by including an elementType specification for a feature whose range is FSArray.
The XCAS and the Xmi serialization forms serialize these as FSArray. The deserialization code, when deserializing these, looks at the feature declaration to see if it has an elementType, and if so, changes the type of the Feature Structure to that type.
This is fine except for two cases:
1) backwards compatibility - this wasn't done in v2. This might be an ignorable difference (except for utilities that compare CASs, and don't accommodate this).
2) (more important) Sometimes feature structures are "shared" - that is, multiple different features might reference the same one. These features might not have the same element type. A not unusual use case is the one where the item being referenced is a 0-length Feature Structure, and the code is sharing one common (immutable) instance of this.
For #2, one possible fix is to examine the "multipleReferencesAllowed" property of the feature, and only do this narrowing of the type if this is false, and the rest of the code hasn't accidentally shared this feature structure with other references.
For #1, perhaps the right approach is to have a backwards compatibility -D flag that (if set) avoids doing this type update when deserializing.