If Amber is already eating pre-killed and you get the temp right, you'll probably have an uneventful switch. I would suggest using a thick (like freezer-style) plastic bag to do the thawing in to keep the rodent dry and help retain the "yummy" scent, and remember that normal body temp for mice and rats is ~98.6, so you should (in most cases) be aiming for 98.6-100F for the warmed temp. Good luck!