Compatibility is limited to the following:
CPU ROM v1.00 is only compatible with a v1.0x control ROM (only 1.00 exists)
CPU ROM v1.10 is only compatible with a v1.1x control ROM (only 1.10 exists)
CPU ROM v1.20 is only compatible with a v1.2x control ROM
CPU ROM v2.00 is only compatible with a v2.0x control ROM

Change:
One of the bugs present in 1.20, and fixed in 1.21, relates to the processing of multiple, consecutive NRPN messages. This is only an issue where the playback of particular GS sequences is concerned. 