I would recommend one step at the time, remember an animal that is well established is in most cases easier to switch than one that is not.

So first I would make sure they have several successful meal (live mice) than I would offer live rats (same size than the mice offered), I would do so for several meals (3 to 5 feedings) before attempting the last switch.

Or you can go from live mice to f/t mice and than f/t rat just remember one step at the time will likely make it an easier transition, going from one type of prey to another and f/t when the animal is currently on live could be difficult or more tricky and take longer.