Thanks for the reply, Sjoerd.
I don't use Time Machine, but rather CrashPlan, and I excluded most Applications from that backup, unfortunately.
I will definitely turn off auto-updates if it's on (I don't think it is) but more importantly backup the app before upgrading although I'm not sure how simple that is. Eg, can you just backup the MainStage icon bundle? What about the gigs and gigs of supporting libraries and config files? This challenge is the main reason I never backed it up in the first place.
Would a Time Machine restore be smart enough to roll back all the corresponding libraries?
If not, the only safe solution might be a complete hard drive backup prior to upgrading MainStage, which is a real chore, especially if I need to restore. They should make rollbacks easier to do, given that we have so much on the line, and they can't guarantee the upgrade isn't going to break things.