I think people can change, if they want to and if they have the resources to be able to do it.

I've watched my DH turn from a bit of a player/ladies man who had never been consistently faithful to a wonderful partner (now husband) for me. He is quite open about the fact that he just had to grow up, to realise there was someone else he needed to think about and that he needed to be a better person to keep me! He is a very strong, independent and determined person and I think those were his "resources" that he drew upon. People who knew him 8 or more years ago but haven't seen the change seem surprised he is now married - I wonder what they'll think when we start having babies

On the other hand my ex was an extremely addictive personality (alcohol, pot, gambling) with what I suspect was undiagnosed bipolar disorder. From what I can see (the joys of facebook) he hasn't changed, despite having lost everything several times over. He will never change until he a) wants to and b) gets some help for the things holding him back.