Recently I ported from Straight Talk BYOD SIM with AT&T network to AT&T.
You need to two pieces of information to migrate to AT&T.
1. Last 15 digits of your SIM card number
Before you start the porting process, you need to setup your pin. So I logged in to my straight talk online account and setup a pin.
Eject your sim card and note down the last 15 digits of sim number printed on it. If your eyes are not co-operating to see those tiny numbers, you can install an app that shows you sim card number.
Go to an AT&T store and give these information and the porting will be complete in 5 mins.
If you are trying to migrate to AT&T via on-line, then it will not work, it will error out saying your number is already with AT&T. So you need to go AT&T store to do it.
If you are moving to AT&T and pre-ordering an iphone 6 at the same time, then you will get an error. So first you need to migrate and port your number and then pre-order your iphone 6.
I traded by iphone 4s and received $200 at&t gift card.