Apple will swap out the phone no matter how old believe me.
They won't if it's some unlocked phone on another network though since they'll be swapping the sims etc.
BUT! Why not try this.
Restore the iPhone to the latest firmware so it's not jailbroke. Find a friend who has an at&t phone and swap out the sim so it's active on at&t. Not sure it will be active though, so maybe try hooking it up to iTunes and see, if not you might need to jailbreak it but don't install any install.app or anything so it looks legit.
Then bring it in. Make sure you know your friends number because they will need it once they rehook up the new iPhone to iTunes to force activation at the store.
Worth a shot I guess to get a swap.