Quick upgrade swap/preorder question

Ok sorry if this has been posted a bajillion times already, just asking for definitive clarification-

I'm on a family plan. I have a 3GS. My # is NOT eligible for the upgrade pricing, but the other person on the family plan is, but currently does not have an iPhone.

My intent is to get the iPhone 4 w/ the other family member's upgrade pricing, then give that family member my current 3GS and take the iPhone 4 for me.

Now, I realize that this is possible. But what's the deal with that and pre-ordering? Do I just pre-order it from Apple as if the iPhone 4 was going to the other family member, getting the upgrade pricing, take delivery @ home, then just call ATT and say what I want to do (take the iPhone 4 onto my # and my unlimited data plan, and swap the 3GS to my family member with the new capped data plan)? There is no need to get another micro-sim, correct?

I guess the bottom line is, is there any need to go to an ATT store if I want to do this? Or can I do it all via online ordering from Apple & a phone call to ATT once it arrives? Thanks!