Here is the full process for me, I am on ATT Next 12. So being on this plan, they do not let you exchange phones, only return and rebuy.
So before i went in, I reserved a new phone online for in-store pickup, no commitments here. Just reserves you one in store to buy then.
Go to the store at the time you picked, You will fully return your current phone for a refund, be sure to bring back your box, accessories and your original receipt, you will need to show them your original order number as well, once returned you will only get back what you paid in tax - assuming your swapping for the same phone (color does not matter).
They then give you the new phone, just like your signing up to add a new phone to your plan, charge you the same tax amount and activate it on your plan with your same number.
That's about it, total process took about 30 minutes.
Again, be cool. Just say your not happy with the current phone (i said my battery sucked and it got hot - which is true) and want to return it, but you have reserved one online for pickup. I always try to scope out the person who I think is gonna give me less hassle in general and try and pick them to help me, not always an option though. You will not be dealing with a genius in this return/rebuy situation, just a regular sales person.