I can understand that you wish to feed F/T if that is what is more convenient for you however I would suggest you to feed the same type prey (Live) for 3 to 4 feeding before attempting to switch.

Once your new BP eats for you with consistency than attempt to switch him.

Remember getting your new snake to eat for you is your priority, switching is secondary.

Are there some snakes that just never switch?
Yes some will not switch, some will but it will take time.

Feeding live is always something that a snake owner should be prepared to do in case their animal did not switch.