Goodness! If UA has a flight on the same day but different time and has EMPTY seats on it, what is the big deal? Give the customer who wants the seat the seat. They were going to fly you anyway and have already paid you. See, this is the lack of customer service that is bothersome. It costs UA nothing to do something like this (unless you wanted double miles, double upgrade, etc. ;-). If the seat goes out empty then they still don't have any revenue and just have one pissed off customer who will tell everybody else about their experience ;-).