Changed for the better

In psychology, you would call that the fundamental attribution error: We, coming from individualistic cultures, tend to stress traits rather than situational characteristics. The concrete situation in the present has a much stronger impact on us than we commonly assume. There aren't simply "nice people" and "mean people", for example, it is our situation which determines our actions.

So what has all this got to do with the topic at hand? I, personally, like some of the ideas of symbolic interactionism, which claims that "the person" as such is never finished, and never just a product of its social environment, but that the mind is a process, constantly in motion, always changing. Our interactions with others and, most importantly, with ourselves are what determines our actions. We actively define the situation and choose which stimuli to take in and which to ignore, but the situation does influence us as a sort of "framework".

So yes, I think people can, and in fact do change all the time. Sure, there are certain personality traits, but it is the situation which brings them out. Especially some, strongly emotional experiences can be life-changing (e.g. illness, death of a loved one, a traumatic experience, a great success in your life). People grow with the challenges they face, they become more self-sufficient, or more bitter, or a million other things. Everyone deals with their life differently.

Jam it back in, in the dark.