Lumia 920 No Contract on AT&T

I'm wanting to buy the 920 form AT&T without renewing my contract, but when I try to buy it online it says that I must pick a plan to go along with it. Right now I have the grandfathered unlimited data plan and I don't want to give that up. I assumed that buying it contract free, I could just buy it and pop a sim in and keep my current plan, but apparently that's not the case, at least online. Does anyone know of a way I can get it from them without changing my plan? Would it be different if I bought it in store? If you buy it from Best Buy contract free it's like $150 more or something, so I'm trying to avoid that. Anybody know anything?