As a general rule, hatchlings are the EASIEST to change prey items with. They are hard wired to eat more frequently and more opportunistically to grow as quickly as possible to avoid becoming prey to other predators. That means in their natural setting, they can't afford to be selective about what they choose to eat. By taking advantage of this instinct, it's quite easy to switch them early on to the prey you would want them to be on for their lifetime.