I've recently taken on a friends hamster, Patrick, he' 2 and half plus. When my friend had him he was going bald. His back half of his body was completly bald expect a few hairs but quite recently his furr is growing back and I was wondering why that could be as I'm not quite sure because at this stage in his life I wasn't expecting his furr to be growing back.
A change to a more suitable diet can help aid in fur regrowth. Besides that, hamsters will lose and grow fur at seemingly random times, so it's really no surprise. If the change is that drastic, I'd assume it has more to do with diet