So this happened overnight:
I didn't even "upgrade" anything!
Do I now have to do a full reset and wait 2 days for all of my music to sync again?
Answer: Yes. Yes I do.
I tried dicking around with using ifunbox to delete medialibrary.sqlitedb but that didn't make "Other" transmute back into "Music". Then I used it to delete all the music, and after iTunes got unconfused it re-synced everything. Maybe this is marginally better than restoring from backup because I won't have to re-enter all of my passwords (iTunes's idea of "backups" being "don't actually back up anything that matters") but it's still going to take 2 days to do the sync.