I'd keep it, but they may have you open the new iPad in the store and swap the sims. If you take note of your account information before you return it you could probably call AT&T and get them to reapply it to your new iPad without swapping the sim, but it would be easier just to swap them.
Speaking from personal experience (an hour on the phone with Verizon trying to switch iPad account to new sim), YES, swap the sim before you leave the store!:
Speaking from personal experience (an hour on the phone with Verizon trying to switch iPad account to new sim), YES, swap the sim before you leave the store!: