My daughters are in college now and, hopefully, one of these days they'll be ready to take on their own bills and make their own choices.  Let's say they choose to go with another company and we agree I carry them until their contract end dates.  Wouldn't that break *my* contract under the family plan, automatically switch me to another plan and penalize me? 

As for the same-day renewal, are you saying if my contract date is tomorrow and theirs are later this year, I can renew them so that we all expire in January 2015?