Good luck
Don't forget to ask for a new "larger SIM-card"... so you can use an older phone (if you have one) until the white comes out.
Also, DO NOT change your IMEI number online and you get to keep your unlimited data, if you have it, worry free.
Rather a very simple process they have made it.
Also, do note I have heard of people's eligibility not updating right away, taking up to 72 hours... I'd call att and verify yours is updated.
Mine was updated and ready for the new purchase before I even got home from the ATT store... so it only took about 15 min, but I was very adamant in store I wanted mine done immediately and not 3 days later.