Or, you can just take your ticket (hopper) to the ap processing center and pay the difference there and get your ap at the same time.
A few FYI's: make sure you upgrade after any black out dates in your trip, and after you've used your early entry. YOu can upgrade on a black out day and still get in that day. I recomend you keep your receipt with you. They will back date your a/p to the first day you used your park hopper, and sometimes they'll question it at the gate, since it looks like you bought your pass days earlier rather then on the black out day.