I'm on a family plan with 3 other lines...I used my upgrade last year for the iPhone 5 and am not eligible til 2014. However, the other 3 lines have upgrades that won't be getting used anytime soon. How do I go about using one of those lines upgrades for my line if I was planning to buy it at Best Buy day of the 20th? Do I purchase the phone at Best Buy and then take it to AT&T where they will take the other lines upgrade for me and apply it to my line? Or what?