The breeder I have dealt with in the past a few times keeps his on mice their entire lives, just feeding multiples when the snake is larger. I have been doing the same with my snakes, since they were raised on mice and seem to prefer them. I have heard that rats are a bit more nutritious, but not so much it is a "you have to switch" but more a "it is preferred you switch". In my opinion, it is more important that the snakes eats rather than WHAT he eats. If he is hunger striking on rats, don't feed them.