This leopard-changing-its-spots analogy is the wrong way to look at things. It assumes that ethics and morality are permanently writ onto our cores, and that to change those values, one would have to become another person entirely. This is a false belief.

As I mentioned in a comment below, numerous studies have shown that a person's ethics are heavily influenced by their present circumstances and environment, not some unchangeable inner nature.

Your rhetorical fails instantly. People aren't leopards, they don't have fixed 'spots.' So yes, most adults can change their value systems any time they choose to. In my personal experience they frequently do. In fact, I've never known a single person that didn't change quite a lot every five years or so.