If your burm is used to taking rats, and only live rats at that you really ought to make the change over to f/t first... then go to rabbits. Feeding mulitple prey will aid in the change.... try offering f/t rats one week. You may find that it takes them right off, most snakes that I have come across will. If it doesnt take the f/t wait a week then try again. If it still hasnt taken the f/t by the third offering offer a live first then for the second rat toss in a f/t. The live prey should get the snake in to feeding mode at which point pretty much anything will get eating.
Once you get it on f/t rats... then do the bait and switch to get it to take a f/t rabbit. Offer a f/t rat first, slightly smaller than normal. Then for the second prey item toss in the rabbit.
You really should get it to f/t first or stick with multiple rats. A rabbit can do a LOT of damage VERY fast. A rabbits hind feet and claws should be registered as deadly weapons !