I agree that best advice would be for your dad to go in with you when you check in. But if he is not available at that time, he could try going to the airport beforehand to try to sponsor the upgrade. I am not sure though that they would pull upgrades from 2 different accounts for the same flight (not sure if computer even allows that) but they certainly could let your dad sponsor to you the 5 500-milers even though again officially it is not allowed.