IPhone :: Adding A Line/ Then Swapping That IP To Primary Line

Is this the same as an upgrade swap? I actually talked to ATT via online chat, and they said once an iPhone 4 is ordered, its stuck to that line. But im pretty sure thats not true.So I want to add a line with iPhone 4 then once I receive her, put it on my primary line, use a dummy phone on the new line, and cancel data on the new line. It works out because I need a new line anyway, I have unlimited data on my primary line, and it saves me some $$ in getting the iPhone at full discount price.

The only thing I was worried about was being able to cancel data, but the ATT rep said I can cancel data at any time. And to order online, I would need to add the single line to my account and order it, once I receive it then I can switch to family plan.Will I need to do the same call ATT and give them IMEI's of everything to swap? Anyone done something similar?