People can do change but it takes a paradigm shift. That is, they have to believe something very different than what they formerly believed, and act on their new beliefs. And seeing how the Paul brothers were always very short sided about pretty much everything, I'm not sure.
Not everyone can change though. Some are just too stubborn to change and some are just too far gone. And there are things people can never change: Their past, issues with their families, their priorities... the list goes on. To be honest, I don't think everyone can change, it's just that we as humans would like to think that people can change but that's not always true.
There are plenty;y of evil people out there; like Hitler. He killed millions and I don't think he could ever change.