I decided to cancel my online pre-order and do in-store instead of delivery (luckily not too many pre-orders at the particular AT&T store I went to), as I figured I would be more likely to get one on the release date.
All I had to do in my case was dial the Premier 800 number and cancel the order. They didn't give me any issues or say they couldn't do it. I received a cancellation e-mail within 15 minutes. Of course my initial order was on the 10th, so maybe that fit within the cancellation window, or Premier doesn't mess around..