A snake that eats other snakes does not have to eat other snakes as a diet, as they have ample ways to convert it if the snake proves to be a stubborn eater.

Like Skiploader said though, this may not always be the best case for the animal, and in different animals it affects them different from taking them off of a diet that they've been dieting for so long. I know like in spiders, though I can't remember which ones, if you take them off their natural diet then their venom is no longer toxic, at least I think it was for a spider, but now that I'm thinking of it I think it may of actually been the poison dart frog.

Sometimes we do what we think is best for the snake, when it's actually the total opposite because it's more convenient for the owner. This is another thing that should be thoroughly thought through when purchasing a snake that isn't on an all rodent diet in it's native habitat.