Okay, so I have a family plan with three lines. The main line is mine, which I used for an iPhone 4 upgrade last year, and is not eligable for an upgrade until November. However, the other two lines have been eligable since April.
What would be the process of upgrading the main line to an iPhone 5 when it gets released using one of the other line's upgrade? Would I just order the iphone off apple.com under say line #2, and then just swap the micro sim cards when it arrives? Or would I have to call AT&T and have them switched.
Order your iPhone 5 under one of the other lines. When it arives, call ATT and make sure they update their systems to reflect that each line has the correct phone and data plan. Given that lines are or will be iphones, it should be a very quick call.
This is done and recomended all the time.
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Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/