I have had this happen multiple even a month or more out. And after failing the offer remained. Most of the time, a call would fix it and get it process (it was $600 each time). However, one time a supervisor refused, even acknowlding the offer was still available claimed UA had the right to cancel at any time even if UA confirmed the upgrade and issue a credit card hold!!! As it it was till there a week later, I got a supervisor that agreed UA was morally and likely legally obligated to honor the offer. I had submitted a DOT complain but became moot due to the second supervisor.