SIM to MicroSIM

I'm requesting the help of the amazing Verge forum members. I have an unlocked Nokia Lumia 800 on the way to me. I recently realized that the Lumia 800 uses a MicroSIM, whereas I currently have an older full-sized SIM from AT&T. If I go into an AT&T store, will they swap my SIM out for a micro? Has anyone had any hassles with doing something like that? Or am I better off spending a bit more money and buying a SIM cutter online? Thanks for any help!