Yes, I know about bak files. But maybe many studio sound engineers do the way you described - not seriously work with unstable system and don’t switch versions in the middle of project -, but in my case - whenever I open any version of Cubase, my muse is right beside me and the job starts immediately 
Which version (5.5, 8,5, 9.0, 9.5) I open, I automatically open one of latest projects and continue to implement ideas. Of course, if project can be opened in earlier version. My fault is that musical ideas are coming everytime and everywhere
I just need to learn myself to not working everytime, but always any testing of versions ends with saved sketch or idea for next composition or song.
Thank you! Backups we do. But they are for crash solving, not in everyday’s workflow