Alright so I already have a 3GS, but my mom has a blackjack. Even though she can upgrade she doesn't want to. So to not let this upgrade go to waste, I want to use my moms upgrade to buy a refrubished iphone 8gb for 50 bucks, swap out the sims and sell the 3g on criagslist. I heard AT&T can tell what phone you are using on their network, so will they care when my mom is on the iphone plan but shes using a blackjack instead? Were on a family plan and she already has a 30 dollar data plan on her blackjack, so the monthly charges should stay the same.