Have you had any luck with the ft? I have switched a few snakes over. Some can be really tough. There are certain things you can do to aid the process. A big thing is when you are switching over you really need to commit to it. If you try feeding ft for a while and he won't take them and you give back live it teaches them they just need to wait it out and eventually the food they want will come along.

That being said I have never done the pre killed in between so I can't say what aspect that will add to the process.