About a couple of months ago my mom needed a new phone. We currently have a four line plan with AT&T and so since she was not upgrade eligible until this month, I gave her my line upgrade and she got an iPhone 5C. Her line is now eligible for the upgrade and so I was planning on getting a 6 for myself using her upgrade. ATT is now claiming that her line is not share upgrade eligible (what the hell?) and saying that only she can buy the phone and use it for her line. I have been on hold forever waiting to speak with a supervisor. But if all else fails and they will not let my line buy the phone, and hers has to, can someone please tell me what we do? Are the SIM cards the same on the 6, 5C and 5? So we could buy the phone under her plan and then just swap the sim cards to give me the 6 (using my line's SIM from the 5 and she just keeps her current 5C)? Also, if we order the phone under her eligibility as a preorder, will that screw with her current phone's service? Sorry for the stupid questions but I've never had a problem upgrading a line before, whether it was mine or one shared from another account.