Once you upgrade a score to Musescore 4 format there is no way to 'go back' to a previous version with that score?
If I take a Musescore 3 .mcsz open it in Musescore 4 and then save it, that's it right? There is no way to open it in Musescore 3 ever again?
I just want to confirm this. Because it is going to affect what I am doing.
Back up files first
Yes. Either create a backup or use Save As the first time if you want to keep your 3.x files.
You can always export to MusicXML from 4 as well and then open that up 3 in case you do need to bring back a file.