Originally Posted by dcgators
Maybe I'm wrong but if you can't support the upgrade, you still have your confirmed economy seat as long as you got a seating assignment prior to getting to the airport. Am I missing something?
With 500 milers, your upgrade is confirmed, your F seat assigned, and your Y seat released into available inventory all before you check-in.
So it is possible to have 8D, be upgraded to 1B, UA sells 8D to someone else, and then you check-in and can't support your upgrade. Since 8D is no longer available, they can't put you back in it (unless they auto-upgrade 8D and if 8D has already checked-in...).
UA might allow agents to make you negative on 500-milers, so if you needed five and had four, you'd go -1 and the next four you earned, one would be applied to the negative leaving you with three deposited.