Personality Change or Behaviour Change?

It seems to be widely accepted that an individual's personality or "character" is largely set by the time that they are five or six years old. At this point the personality is largely fixed and all that an individual can do is modify their behaviour to deal with new situations. Whether this is because of inherited characteristics or experiences gained in those first few formative years is open to debate.
This certainly seems to be true for me and I feel a real sense of continuity in my being which has lasted through my childhood and adulthood through all of the challenges that life brings. To myself, I am still the same person as the nervous small boy who didn't want to enter the classroom on his first day of school. I am still the same person as the one who pined for weeks after he was dismissed by his first love.
The fact that I am now a writer of fiction gives this a particular resonance. It's often said that a central tenet of fiction writing is that the main character (or characters) should experience change during the course of the storytelling. What I find in my writing is that the main character will experience change and will develop behaviours and strategies for dealing with that change while they stay very much in character. By the end of the story the world around them may have changed but they are still, very much the same person. It's likely that all of the characters that we develop in writing fiction end up being ourselves, in various guises, so I suppose it is only natural that this sense of continuity of personality will apply to these fictional versions of myself.
I'm somebody who is largely happy with myself as I am but there are certainly some things about myself that I would change if I had a second chance. In particular I would do more to show my love for and appreciation of my parents. So, for one thing I would say something more to comfort my mother as she sat on the sofa in shock after the death of my father, while a priest clumsily tried to elicit information about him for the funeral service. This is where my sense of continuity in my character lets me down and I realise that to achieve this I would need to be reborn as a different person to the one that I am.