Switching is about tough love, if the animal does not eat what you offer than try again the following week and the one after that without offering anything else if it does not eat.
This require the animal to be of a good body weight as it could take MONTHS with no food before switching.
And remember males do not require medium rats a small rat weekly for a male is way more than enough especially if you want an animal feeding with consistency.
Of course you need to take in consideration that not all of them will switch either and there is nothing wrong with that, if that is the case feed what works.