If this has been the rule, then I just found out the hard way.

On Friday I have flights from BRU-IAD-SFO. The IAD-SFO segment cleared for the upgrade. However, UA says that because I did not call back within 24 hours of the upgrade clearing, that they took the upgrade away from me. What?? My estimate is that the upgrade cleared sometime on Saturday. I did not check united.com (or my email) over the weekend. And, I also did not receive any telephone messages about this from UA, as they stated to me. However, because this morning My Itineraries was showing KL, I called.
Is this 24 hour rule real?
(I was using SWU's, and I have never run across this before).