switch to "by the gig"

Why doesn't this work?

"We're flexible

This line is on Unlimited right now, and you can switch it to By the Gig as long as you haven't already used more than the available By the Gig shared data amounts."

But I cannot switch:

"Something isn’t right, so we can't process your change. Wait a few minutes, then try again."

This has been going on for over a week and, as too often is the case, calling in to support does no good: they run into the same problem and have no idea what to do.