Doing it on contract first and then coming back a day later would be no good. Especially if you open it, as they will not be able to resell it to you. The store may also not want to do it for you. I had called before and asked specifically if they would and was told yes, so when I went in to do it, they at first were kinda like no, it won't work, but then went ahead and did it for me once I told them I spoke with someone there who said I could do it.
They also said they had to warn me that once they return the phone, it goes back to inventory online and someone else could snag it. I told them I was a gambler and willing to risk it. Lol.
As to whether or not it will work is really dependent on ATT. If they reset your eligibility quickly, you will have no issues. If not, you may be sunk. For me, it reset almost instantly so it worked out fine.