If you were already with ATT and upgraded phones, you'll probably get a refund for the phone but have to pay the early termination fee if you switch carriers (I think it's either $325 or $350, at least if you have a smartphone). If you decide to come back to ATT, then you would be able to get the normal contract pricing on any phone since you would be setting up a new account all over again.
However, if you do the switch within 30 days, you should be able to waive the early termination fee (may need to argue it a bit, but they'll take care of it if you speak with customer service), but as for phone pricing I'm not sure. I would assume you'd get the contract pricing/instant rebate so long as you return the phone you just got (also assuming it's been within 30 days). You'll just have to pay the activation fee when you get your account back.