Looking to switch to T-Mobile from AT&T. I currently have 3 lines. Two lines are grandfathered unlimited plans and the third is a 3gb a month (father hardly uses 1gb). I pay $238 a month, my wife's 7+ is on Next and is $32 of that. I live in SE Michigan and it seems like the coverage should be good. I notice T-mobile will pay the remainder of her payment plan up to $650. She owes roughly $300, this brings me to my question. What stops me from picking up a new phone on next before I transfer? My thinking is I'd get a free $350 off the phone. I imagine I'm missing something huge such as AT&T cancelation fees. Side note can this be combines with the Bogo plan for Iphones.

Thanks everyone!