I think this would be easiest done by using Move Assigments located in Ministry -> Children's Ministry (or your ministry name) -> Participants -> Move Assignments. Here you can select who to move (or not move) and where to moved them to.
I have been giving this some thought for our promotion Sunday and think it is how we will do it.
Blessings 
Renee Conners
Katy, TX
www.parkwayfellowship.com