I once did a domestic upgrade with miles from my dad's account and the agent just asked me if I wanted to use the card that I purchased the ticket with to pay the co-pay amount (the card was in my name, didn't have my dad's name on the account). Why I "wasted" his miles on a domestic upgrade? Becuz there were only like 18k miles in the account and my dad flies exclusively with United now (he's premier Exec with them) and there really isn't anything you can do with 18k miles (15K of it was soon to expire, the other 3K was non-expiring miles from the primitive days of AAdvantage). Even better though was the 3k miles left in his account were the old type that NEVER expire (pre-1989, that was while the soviet union was still around...).