It seems that everyone runs into this problem eventually. Why can't they just keep the SIMs the same size?!
Personally, when I upgraded my smartphone that had a standard sized SIM to one that uses a microSIM, I didn't want to deal with AT&T, either. I just went on eBay and purchased a microSIM cutter. It's as easy as using a stapler. It was under $4.00, too. I'll be using it again pretty soon to upgrade another phone.
If you insist on going into a corporate store to get your new SIM, all you really need is the SIM itself. It would be helpful to bring in both phones, however. If they say something to you about the $0.10 per minute plan not being compatible, then I would just suggest buying a microSIM cutter and doing it yourself. They will know that you will be using a smartphone because regular non-smartphones do not use microSIM cards. Chances are that you'll probably be okay. Just don't say too much if you can help it.