Are you using e-tickets or paper tickets? Paper should be easy enough. Just hand in the coupon for the class you've been accepted and return the other coupon for a refund. E-tickets on the other hand may be problematic. It could depend on the efficiency and willingness of the gate agent and as you mentioned time constraints coming down to the crunch of clearing the wait list and closing the flight. As far as I know, its doable, but may prove a hassle. Additionally check UA's policy about listing twice for the same flight.