Its a transfer of service. Its easy. Call customer care tomarrow either 611 or 800-331-0500 and ask for the transfer of service department Northeast region.
One of you picks whos going to whose account, then they ask the person whose account that is becoming the family plan if they are willing to take over the financial responsibility of the line and the person moving has to agree to release their financial responsibilty of the line
Contract dates and everythign stay the same.
Thats all you need.