I'm just a happy camper. The UI for the upgrade is really lovely, and it all went perfectly for me. I'm very happy with the font resizing, and it means I can now read (many) Calibre-converted PDFs very nicely. It's very good to be able to hide the 100 books (which I put back into the device before upgrading). I mean, it's nice to have them in there, now that one doesn't have to stumble over them all the time. Maybe I'll read a few.
It would be nice if the desktop app thingie was all fixed up, but then we can look forward to that.
Lots of appreciation to Sameer H. and the others at Kobo for staying in touch here.
Oh, and of course all of you wiser people were right; the books really do look much, much better when the stylesheet gets left in there.