Thank you for the solution. I'm relieved to rediscover the notebooks I thought I had lost.
Re. "You don't need a warning, there are release notes": I don't have time to read the release notes of all the apps I use, and I doubt most other people do either. Evernote is not the center of my universe.
I work for a software company. We never do anything that would affect customers' data without sending them other special communications in addition to release notes, for exactly this reason.