I current use a iPhone 5 on an AT&T family plan, which I upgraded from a 4s last October. I will buy the new 5s tomorrow, but not from AT&T paying for $450 for another early upgrade, instead, I will buy an unlock T-mobile 5s from Apple store for $649. This way, I can qualify for the $200 upgrade next year. My question, If I pop the sim card from 5 into the new 5s, will At&T find out I use another iphone and charge me the $36 activation fee?