For award and upgrade tickets, average and rule-abiding UA agents will require the corresponding inventory space to be open on the new flight before they will confirm you into the premium cabin, otherwise you are confirmed in Economy with a waitlist for "upgrade". A good agent will work with a supervisor to override the system and confirm you into the premium cabin, but that requires them to get a supervisor to approve the override, and that's usually the hang-up (not HUACA).

This happened to us on a paid ticket that was upgraded into R (back in the days). We had an IRROP in NRT that would have caused us to misconnect in LAX. The UC agent refused to confirm in business class for the new flight, apologetically (as NRT agent does), because R inventory was not available. It took 3 calls to the 1K line to get a sympathatic agent, and an existing waiver with cabin-to-cabin rebooking, to get it rebooked into business class.