I believe you're trying to explain the XG Native vs XG Basic map situation.
Fuckle your seatbelts, this is one wild ride.
Before I get too carried away into esoteric MIDI talk, let's discuss terminology for XG devices. Each of the 600+ sounds you can select via program change / bank select LSB / bank select MSB is called a "Tone". Each is comprised of two layers, each called an "Element". Each element is comprised of collections of "samples" that are mapped to specific "regions" of the keyboard, akin to how multisampling works in, say, ImpulseTracker.
To clarify: Starting with the MU100, the map for MU-series modules changed the default tone for a handful of the XG "basic" tones (or those that do not require a bank select to access) to reflect some of the additions to the ROM. However, all of the old versions of those tones still exist at Bank Select LSB 127, and the new version is duplicated at Bank Select LSB 126.
To recap: LSB 126 is the newly added GM-compatible tone for the MU100, whereas LSB 127 is the original version that would play on the MU80 and MU90.
All that the Basic map does is make it so that the GM tones pull from LSB 127 instead of LSB 126, making the instrument map function less like the MU100 and more like the MU80.
These are not the changes that are problematic for playback. Be warned: we're going full throttle MIDI past this point.
Unlike the MU90->MU100 changes, the MU80->MU50 transition is unique in that the MU80 was released first, and the MU50 was a stripped down version of it.
As such, it lacks some alternate tones entirely. The clearest example of this is Synth Strings 3 on the MU80 and MU90+, which is completely absent on the MU50. However, Synth Strings 4 and 5 are still there, and their names were not changed. Another classic example of this is that the reverse cymbal sound was changed for the dance drumkits from RevCym2 to RevCym1.
I hope you're still with me because this isn't the only change that occured between the MU80 and MU50. The MU50 contains additional sample regions for a good number of elements. Mostly found on the high notes, these don't add much to the elements, but it is still a change that breaks playback compatibility.
But where it gets MONDO fucked up is that YAMAHA decided that the MU80 should be the basis for the MU90, but instead of using the MU80's elements, they used the MU50's elements. This means that the MU80 can't play MU50 music 100% accurately, the MU50 can't play MU80 music 100% accurately, and the later models can't play MU50 OR MU80 music 100% accurately.
S-YXG50 is 100% true to the MU50, with the exception of the GS mode being licensed, DOC and Performance mode and the UI being absent and the adjustable polyphony limit.
Currently working on a compatibility table for XG device playback.