Hi,
Sometimes you get lucky with the switch - a couple of feedings ago I switched three snakes over from F/T mice to F/T rats with no trouble at all. It all depends on the snakes but I would always just be prepared to eat the cost of two uneaten F/T rats for the first week of trying and probably for the next feeding after that as well before trying any tricks.
The extra weeks hunger might help persuade them on the second week.
I never feed any of my snakes anything larger than a small rat but I would try and get a rat about the same weight as two mice or even slightly smaller for the intitial switchover.
Just give the head an extra blast with the hair drier to see if that makes them a little more keen.
dr del