IPhone :: Activation In-store Required For IP 4 Subsidized Pricing?

I know that purchasing an iPhone at the subsidized prices will result in the extension of one's contract but how exactly would they see to it that this is done w/o forcing you to activate in-store?I ask because I'm leaving for Argentina soon and want to get the iPhone 4 at the $200 price point before I leave (phones in Argentina cost millions of dollars ). I plan on jailbreaking and unlocking it whenever the relevant software comes out (along with doing the whole cutting the SIM thing) but in the meanwhile, would I be able to continue using my current iPhone (first gen) on the line that I am upgrading?To lay it out, there are two lines on the account. Both due for an upgrade. Mom will upgrade her line to new iPhone and will activate and use as normal. I upgrade my line to new iPhone but don't activate and instead use current iPhone on that line until I leave and then give it to a sibling to take over the number (I don't think I have a choice but to give up my number). 1st gen phone doesn't have 3g so I have $20 unlimited plan. Agent already told me that an upgrade to the new iPhone will cause me to move over to one of the new data plans but I'm still unclear as to when I would have to choose the plan. At home via iTunes or what? Do they end up charging you the full amount of the phone if they find that you haven't upgraded your plan to one of the new ones (and indication that you aren't using the new iPhone yet)?I was already told that in the event that I lose my iPhone 4 or it gets broken, or whatever, I can simply use another phone with an ATT SIM (or swap the micro-SIM and use an adapter), give the IMEI number and they'll register the phone and change the plan accordingly.

Could i continue to use the old iPhone w/ that line? I'm assuming that I could while waiting for the phone to arrive so why not after it gets here? I basically don't plan on touching the new iPhone until I get to Argentina but will still obviously be fine with extending the contract for another 2 years (is that all they care about?) since the number will be used by someone.