Yeah, this isn't an easily solvable issue. You'd normally copy the app group, change the field type to BigInt, then add the new App Group to the folder definition - but I don't think you can map a 'bigint' AppGroup field to an 'integer' folder field.
This might be a good Enhancement request. I've also seem this problem when trying to combine AppGroups that are copies, where one of the date fields is a standard 'date' field, and the other is a 'date/time' field.
-JD.