i would continue to try to switch them to rats, as they do offer better nutrition then mice and are cheaper in the long run. That being said, I have 2 girls that despite trying for 6 months will not switch from mice. In that time they took no more then 2 rats a peice but will eat 4-6 mice a sitting. Best of luck whichever route you go. Scenting a rat with a mouse or bedding from mice can sometimes encourage them to switch.