I had a stubborn female that only took mice as well. I did two things for her. The first was advice that Kevin McCurley gave me. Take some mice bedding (just a small handful) and put it in her enclosure. Watch for her to start displaying hunting behavior - actively flicking her tongue, searching for food. Then drop in a small rat.

Worked like a charm the first time, and then she wisened up.

My most successful way to convert to rats was essentially "tough love". For the snakes that had good body weight, I simply stopped offering mice. I'd offer a rat, and if they didn't want it, the rat was removed - and no mouse was offered. The next week, rat offered, refused and removed - no mouse was offered. The longest hold out was about four weeks.

The picky female above does insist on white rats or beige hooded (guess close to white), but all the others who eventually got hungry enough to take the rat, all take rats every week now.

So, in my experience, the method that worked best was tough love.