I have AT&T right now, im on a family plan with 4 lines on it. Would i be able to upgrade from my normal $15 phone to an iphone and stay on the family plan and pay less or would i still pay 60/80/100 per month. And yes, i am eligible for an upgrade. I saw a plan line for existing AT&T customers that costed 20,30,and40 instead of 60 80 and 100. help please thanks