How old are your JCPs? I had a jcp that was fed rats, then i switched to miss because i screwed up an order and then as soon as the mice were gone he took rats. I was lucky. He wasn't picky at all.

Im guessing they are pretty young because feeding 6-8 mice can be pretty annoying and you don't seemed too annoyed. Switching can be done, but depending on the individual it can be extremely hard. Just as said before try prescenting the room. Or maybe just offer the rats straight up. Who knows, maybe they'll take them. If not then scenting is going to be the only way and it could take maybe a year if not more of scenting to have them completely switch.

Goodluck!