I had a suspicion this is what was happening on a flight I was watching for weeks. I don't like this as I want to confirm PZ at booking rather than hope I magically clear within 24 hours while I can still change/cancel the ticket. I have great success using GPUs because I only use them when I can confirm at booking (getting creative with routing and +/- 1 day of desired travel). The only way this is helpful is if UA is slightly more generous with the number of upgrade space they make available.