In terms of compatibility, there are three main aspects that need to be considered.
API Contract Compatibility: MTJ API will keep the contract compatibility on 1.1.x/1.2.x release.
SDK Workspace format compatibility: Currently there is no plan to change the format that the imported SDKs are stored.
MIDlet Workspace format compatibility: MIDlet projects format is extended to support LIBlets development and new Code Validation Builder. In the same time the old 1.1.x/1.2.x projects are still supported without modifications.