I'll thaw a rat in the fridge 12+ hours, then leave it in a disposable plastic container on top of the rack for an hour or so to pre-scent. I'll then blow dry it (standing in front of the rack to help with pre-scenting) a bit to heat it up, concentrating on the head. I always check with the temp gun to make sure it is not too hot. I wait until I see him with at least his head out of the hide before offering. Seems to work well for me.
I can't help with switching, though, because mine was already taking F/T when I bought him. Good luck with switching.