You need to understand that switching prey type is already difficult in most cases and that trying to go from Live mice straight to F/T rats could be even more difficult.

My advice take it one step at the time.

Switch to live rat, try offering a rat the same size than the mice currently offered, this make the transition slight easier.

Then once your BP has switched and has successfully ate rats 3 to 5 weeks in a row, switch to F/T.

It can take time and require tough love so if your BP does not take the prey offered, do not offer a live mouse instead try again the following weeks.

Of course tough love requires an healthy animal with a good body weight that can handle missing meals for weeks even months.