One thing to keep in mind, and it's been stated on this forum in numerous posts. Rats have much more protein for our pets than mice do. Rats are the preferred food for our snakes when it comes to nutritional value.
Now, there are ways you can try to wean your snake to rats. As was mentioned, you can bag your mice with the rats and let the scent of the mice be absorbed into the rats. Another thing you can do, and it takes a pretty strong stomach to do it this is, you can basically open the skull of a mouse and rub the brain matter of the mouse on your rat to give the rat a definitive scent. For some reason pythons go crazy when they detect the scent of brain matter. Again, this is a gruesome and messy task that is not for the faint of heart.
Hope these tips help.