Do you have access to rat pups or weaned rats? Balls can certainly eat mice their entire lives, but it does get inconvenient as you will eventually have to feed multiple prey items at each feeding instead of one appropriately sized rat. Not to mention, mice stink.

As for when to make the switch...I try to start my babies on rat pups right after their first shed. Maybe a quarter of them will take a frozen thawed rat pup as their first meal, and they will never taste a mouse.

If they show no interest in rat pups, I'll try a live mouse hopper. That ususually gets the rest of them feeding. Once they start feeding, they seem to get really hungry, all the time. After a few live hopper mice meals, it's usually easy to get them to take a frozen thawed rat pup. Just feed them a mouse hopper, give them a few extra days in between the next feeding and offer a rat pup. They usually take it without hesitation.

If you have problems switching from mice to rats, there are several tricks you can try such as scenting the rat, using mouse bedding to get the mouse scent going, smaller enclosure sizes, feeding later in the evening, using different substrates or adding wadded up newspaper to their tubs to give them a better sense of security