Has the way we enter MIDI notation of a new song into OnSong app for each song changed with the new version of BBMO? Can I continue the way I have always entered MIDI notation as usual for years or do I have to change with new BBMO?
Short answer: You can continue as before 
Longer answer: If you for any reason change the order of songs within folders or the order of folders then you can’t use any fixed midi definition like what the OnSong uses 
But as said, the good news is that you can use the same notation as long as you always append the songs / folders to the end of list 
So even though we have BBMO that replaces the old BBM the BB is still the same (no new firmware). So until BB is drastically changed with a new firmware (likely never will) the same midi definition works 
Thanks