Alltel uses CDMA technology and the Atrix ix a GSM phone. Therefore, you can't upgrade to the Atrix on Alltel as the Atrix is not compatible with CDMA.
I'm guessing you didn't notice this was the Alltel to AT&T transition forum. Anyways, from what I hear, they will let you keep a grandfathered plan with the Atrix or the other new HTC "4g" phone. I plan on picking up one myself, but I'm just gonna buy it outright and not get myself back into a contract. I would be careful where you pick it up though, as it may be harder for a 3rd party retailer to keep your current plan. I'd get it straight from AT&T themselves. Plus, wait 30 days after you get your replacement phone so you don't have to send it back, then sell it to make up for some of the cost of the Atrix.