I don't think you ever change him completely. His attitude is part of the whole package.
That being said, there are major factors that absolutely contribute to player behavior. One major one is environment, and I don't think you there has been a much worse place for Cousins over his career than Sacramento. The other is age/maturity. Many "headcases" do mature with age (Rasheed Wallace, Zach Randolph, Ron Artest etc.) They don't change completely, but they do get better, especially in better environments.
While I can't blame anyone who insists that Cousins is who he is, I don't think it's crazy to assume we've probably seen the worst case scenario for Cousins and that there's a good chance he becomes a more dialed down version of himself in the future. I think that's a more than fair argument and should definitely factor in the decision of any team thinking about acquiring him.