Good news: the only thing that broke was the color scheme on my panel bars, which changed to black text on black background. I fixed that right away, and I'm still running a pseudo-Gnome2 desktop.
So, I'm happy with it. Besides, this is an LTS release, so if I don't want to bother updating I can keep it for a couple of years.
Just a bit later: OK, one bug. For some reason the permissions on the directory
had been reset to 555, meaning I could read or execute files in the directory, but not write to it. This meant that backintime wouldn't back up my disk. Fortunately, backintime has an excellent error log, available from inside the program, that told me what was going on. (Unlike Google-Earth, which is still failing to launch in 64-bit mode after all these years.) I just ran the command:
chmod 755 $HOME.config/nautilus-actions
and all was well.