I just went through this with Best Buy. I returned a phone October 2nd and the guy told me it would take 3 days for reversal. Cool, I didn't plan on getting a phone. I had some unfortunate things happen about two weeks ago and needed that upgrade. WEnt into the store to get the 4S and was informed I didn't have an upgrade.
Went to Best Buy, had them print the receipts and such again and told them I needed them to reverse the upgrade. They called AT&T and then gave me the 3 days BS again. Went to AT&T the next day and they told me that I had to go back to the store I got it from and tell them to call in and have the upgraded reversed and that it was an instantaneous process. After trips to two different Best Buys, I returned to the Best Buy I bought it from and demanded a manager. The manager mimicked what AT&T had said and had one of his reps get on the phone with AT&T and I waited about an hour or so but the upgrade was reversed instantly and I got a gift card.
So, you're going to have to go to Apple and have them phone AT&T and tell them they want to reverse an upgrade because you returned the phone. That's going to be the only way. AT&T corporate reps can do it, but they will get in trouble if they do it themselves so they don't.