my sister has att, and she is eligible for an upgrade. she said she will let me use it to get the new iphone, if i paid for it, which is fine. but i don't use ATT, i use tmobile. will they let her buy the new iphone at the 299 dollar price if she doesn't change to an iphone plan on the spot? i mean, is that a deal breaker? either you change your plan when you buy the phone, or you don't buy the phone? or can she just say she'll change it later, and extend her contract or whatever it is, and get the phone?