The way that I like to view this question is as follows:
We are born with our 'True Nature'; the very basic essence, the beautiful natural core of who we are. In an ideal world, through the natural flow of energy through all aspects of our experience, IE., interpersonal, intrapsychic, spiritual, etc., (Sensations, Images, Behavior, Affect and Meaning - SIBAM) we continue to grow and evolve more and more into this true natural free flowing beautiful self.
Due to environmental stress, 'small' and 'large' traumas, 'attachment' disruptions, Autonomic Nervous System adaptations and the like, over the years, many 'layers' are added to and tend to cover up this natural core. Through the process of growth and change, my experience is that we 'shed' these unnatural, unwanted, negative aspects of 'self' and become more and more that beautiful self that we truly are!
I think some people 'fear' the idea of 'changing our personalities' because our experience of self, of who we ARE, is tied up in and interwoven with those negative aspects. One fear is that we might 'lose' the sense we have of ourselves if we 'change our personalities'. This could not be further from the truth. As we 'shed' the unwanted, unnatural aspects of self we 'picked up along the way', we 'become' more and more who we really are. We do not lose ANY aspect of ourselves that we do not want to lose. And, more good news, This is a natural process. We do not have to work at all at deciphering which parts to keep and which to let go of. Our Autonomic Nervous Systems simply, naturally shed, or in the language of 'Somatic Experiencing', 'discharge' the previously 'stuck' negative energy that was manifested as unwanted aspects of our personality, and thus interfering with the natural flow of who we really are.
Just to 'piggy back' on this theme of fear a bit more;
On a purely physiological level fear can not only create but play a significant role in perpetuating pain cycles and it can manifest on any and all levels of 'SIBAM'. I have my clients do the following experiment in session; Think a negative anxious thought about the near future like, "What if..., (what if I get into pain later) and just simply notice the reaction in the body. You will notice almost global Tension throughout. Now change that thought to a more relaxed, "If I get into pain later, I'll deal with it THEN, not now". Most people will notice, if not full relaxation, at least a significant reduction in the Tension. Let's remember what the first word is in Dr. Sarno's diagnosis, 'T' MS... and since fear and LOVE are incompatible, the more we love (ourselves, others, the world, life, etc.) the less fear we will have, and thus the less pain.