You got a lot of good advice from the previous comment all I can add is use a hairdryer after you use water to dry then up and warm them at the same time. Focus the dryer on the head for a while to heat that up and see if that helps. I've had to switch a couple and some transition smoothly while others are stubborn but it takes a lot of patience. You can also just defrost the rat at room temp them heat up with a hairdryer to heat the rest of the waykeep us updated hope it works out!
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