Yeah, it's pretty easy to cancel a preorder.
Just go in, and say you wanna cancel. We're pushed so hard for preorders that almost every one of us register monkeys will ask if you want to shift it over to something else, or ask why you want to cancel. (and yes, someone coming in and canceling a preorder counts as a negative mark for the day, so we can get in trouble...it's retarded.)
Don't take any of our shit. If we ask, either *politely* decline, or tell the truth. I for one would much rather hear "Yeah, I got it from Toys R Us when they broke street date" than "Uh...my brother like...already got me one...yeah."
Then again, it's about you, not about the register lackeys. Don't take any of our shit, get your money back.
Edit: About the calling on preorders, we don't do that manually anymore. It's all handled by corporate, and you get a pre-taped message from "Aeris" (PUNCH. IN. FACE.) that the game is coming out. I'd rather go back to the manual calls. =\