🙂 I never said that it was going to be easy.
But there are some damn clever people in this forum and I'll bet that more than one of them could help you write that query without it being too complex to maintain. I like the idea because it solves your problem in one hit rather than two and (potentially, if a set-based solution can be found) avoids slow row-by-row updates.
Anyhow, sounds like you are well on the way to solving it & I'm guessing that performance is not your primary concern.
Phil
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.