Don't feed them for a few days. Then try introducing your new food you want to convert them to. If they don't go for it, wait a couple more days and try again. If more than a week goes by and they don't eat, feed them a couple feeders and repeat. They will eventually accept the new food as a fish won't starve itself to death.
Beef heart and other high protein foods are good for growth. (sinking carnivore pellets etc)