There are tons of tricks to switch ball's from mice to rats, but your gonna have to have a very determined attitude because it can be very difficult to switch from mice to rat's.

Have you tried putting some mouse bedding in the front of the enclosure and than dropping a rat in?

Unfortunately, there are some ball's that never make the switch to rat's. I have never had that problem so far and hope I don't as I couldn't imagine feeding my breeder's 4 or 5 mice at a time.

Are you trying F/T or live?