I finally (after about 15 minutes on hold) got through to another united.com support agent. She's saying something about the agent who made the change not reissuing the ticket (oooops). She said there would be a fee for doing it over the phone (even though united.com wouldn't let me change it because I already changed it twice), and a fare difference (even though H was available and that's what I was originally booked in). Hold time for her to research is about 3 minutes at this point. I'll update when I get more info.

I have never had a problem with online checkin. 45 segments this year so far...no problems. I guess I'm lucky.