When I switched from live to f/t, all I did was buy a f/t and offered it with my hemostats in front of the snake's face. Chomp, constrict, swallow. As easy as it could get. Not quite the same as switching from mice to rats, but I think a bit of theory still applies.

You probably have only two options to try out first if you eventually want your snake feeding on f/t rats, switching from mice:
1) offer a live rat
2) offer a f/t rat

live rat might get a better feeding response, but who knows. If your snake is a food feeder, hungry, in a good mood, not stressed etc. I'd say you have a decent chance of him taking a f/t rat if offered.

If the snake refuses.. well that's when you start needing to try different things.