So the website said the date had to be in the future. It was an error that prevented me from doing the upgrade. So I called in, and the first agent said it had to be done at least 24 hours before departure. So I called back, and another agent talked to ticketing, who confirmed that.

So in my case, it's not that I gave up before trying . In fact, I only gave up when I had to board my own flight
Unfortunately, by the time I landed and saw your replies, the flight my friend's on had departed

I'm going to have this issue again on Friday and maybe Saturday, so what should I do if I run into the same problem? I'd really like to upgrade my boss, and his flight is C8 right now