I have a family plan with AT&T, Line 1 (my line) - Just upgraded to the iPhone 5S with another two year contract. Line 2 (mom's line) - Two year contract will be up soon and currently have a none smart phone. My question is that when the iPhone 6 (1 year later) is coming out, I would like to sell my 5S and use my mom's line to upgrade for the iPhone 6, but she will still keep on using her none smart phone. Is it possible to do this? Keep in mind that I have the grandfathered unlimited data plan that I don't want to get rid off on my line. Thanks.