When upgrading 10->11, or 11->12, and after running the new version initially with bootstrap=true does the cms make non-revertible changes to the structure/content of the database?
i.e. would this now render the db unusable with a previous version of the cms?
Yes. You cannot use old version with changes made by newer version. So, create backups before doing this.
As others have said, yes, once you upgrade there is no going back.
From personal experience, trying to run an older version of Hippo on a newer database will irrevocably damage the database. (it was an accident, I swear)