If what truciet is saying is correct, if you were to call AT&T after you activated your phone, wouldn't the upgraded line in which the purchase was made SIM be 'dead' at that point, so how would this "switching" numbers around work exactly?
The thing is, you DON'T activate it.
What I did in the past and what I plan on doing on June 24th is:
Option 1: Work not busy
Preorder for home delivery at my work address.
Once I get the phone, I will take the phone to the AT&T down the block to activate the phone using my line. They will give me a new sim card and they will not deactivate the upgraded line's sim card.
Option 2: Work busy
Once I get the phone, I will call AT&T on my lunch break and just call to activate it. I hate doing this because I will have to read them the sim card's serial