I preordered thru apple.com and paid an early upgrade price on a 2 year contract (AT&T). I had wanted Next, but for whatever reason, Apple is not allowing Next sales over the web in NY state (and a couple of other places).
So I got my 2 year contract phone delivered, and waited a few days til the launch dust settled, then went into an Apple store, staked out a friendly, smart looking employee and said I wanted to return my phone, then have them sell it right back to me on the Next plan.
After a little hesitation, mostly based on the belief that it couldn't be done, I got the employee to ask a manager to try to make it happen. The manager was cool too, and he referred us to another employee who had successfully pulled off something similar.
They all said they weren't big fans of doing it, as it's a slightly drawn out process, and that other stores may not even touch the situation. Apparently scalpers are buying any phone they can...when the no contract phones ran out of stock, they bought on contract phones, Next phones, Edge phones, whatever, then returned them to get them "upgraded" to no contract by paying the difference.
Once the employee saw my device list in iCloud and my email address @me.com, he figured I wasn't a scalper, and he went through with the operation.
It entailed returning my phone, then reselling it to me, with a different SIM, and manually entering the IMEI or something in his activation computer.
The whole process from walking in to walking out with a working phone on Next took under an hour. My upgrade eligibility was reset on the spot.
It can be done. Just gotta find the right combo of people willing to do it.
And if you bought the phone directly from AT&T, it's absolutely easy and no problem to switch, according to a senior level rep I spoke to.