There's absolutely nothing wrong with a snake being a mouser her entire life.
She doesn't NEED to move to medium rats, if mice are sustaining her. YOU may need her to move, but SHE doesn't need to move.
I would recommend small mediums, or small rats. Ask for some mouse bedding at the pet store (in a paper sack is fine), and then put a handful of the mouse bedding in the front of her enclosure. When you see her start actively looking for the source of that scent - drop in a small white rat. She should be "turned on" by the scent of the mouse bedding and then when presented with a live warm rodent will immediately hit on it.
If that doesn't work - then you have to use "tough love" (only if she's in good body condition). Offer rats and only rats each week. No more mice - until she gets hungry enough to take a rat.
If she loses significant amounts of weight, you'll want to offer her mice again. Don't ever extend tough love if you have a particularly stubborn animal to the point of risking your animal's health.