as everyone else said and is posted on the upgrade guide on this forum..
upgrade swap - if you do it online order it for the # with the upgrade, but when you get it call at&t, tell them what you are doing, they will update your SIM & IMEI #s then you can plug into itunes to unbrick..
if you do it in a store just tell them you want to use the other upgrade and they will do all that for you..
the main account holder must be present in store or you must know the last 4 of their ssn if you do it over the phone
upgrade swaps are probably more common then normal upgrades, when I worked at cell phone reseller we did them on the regular basis