Snakes only get stuck on live, they don't get stuck on f/t. So if you're switching from live to f/t, your snake might need some time and tricks to get him to eat.
But if he's consistently eating f/t, he will have no problem eating live again (if for some reason you want to do that). But you don't want to do that, because he might get stuck on live and get picky about f/t again.