Originally Posted by zombietooth

No, your tickets will still be good. You will just lose all of your miles, your upgrade instruments, and your status.

This happened to a friend of mine, and they just flagged his account and name so that he would no longer earn miles when he flew UA, but he still completed all of the trips that he had already booked. It did really suck for him because his status went from 1K to nothing, so he wasn't eligible for UGs.