the only argument i have to offer about simply putting in a new sim card versus AT&T activating you is AT&T go phone.
we have had issues with AT&T unable to unlock the phone, because the phone was not used with a previous account when we simply moved the sim card over.
when we went to the AT&T store to formally ask that the new imei # gets put on the account, they looked at me like i was crazy
after this was done the unlock request was immediately granted.
prior to walking into the store i was denied 2 times, for the same reason, no activity tied to a previous AT&T account. i went to the AT&T store, made them put the imei on our account.
that same day we submitted a new unlock request and it was granted in 2 days
if you move the sim card over, see if you can log on to your account and see if the new imei is on your account.
if it is, then you will get credit for using this phone on their network for an unlock in the future.
the story about how you don't need to pay for an activation fee and you can simply move the sim card over may have consequences when its time to unlock your phone.
^^ this post was limited to how AT&T go phone tried several times to screw me out of an unlock and is not to meant people with postpaid service