Point taken!

But I didn't know that when I started this thread, and it turns out that the problem wasn't one of a lack of compatibility, but a bug in the Mac implementation of BIAB which was revealed by an update to MacOS Sierra.

I still believe that backward compatibility is a desirable goal, and also that it should be possible to "Save As" an earlier version, albeit with loss of some features.

I'm thinking here of sharing files with other musical friends for rehearsal purposes where the friends might not wish to shell out the cash for RealTracks etc, but would want to be able to transpose for their instruments, play with a MIDI backing track, and create a PDF lead sheet.