That’s a complicated question to answer, since it’s an impossible thing to quantify. The short answer is that if I did not believe people could change their personality significantly, I would not have become a psychotherapist.
Although there is a clear diagnostic distinction between personality disorders and other disorders, I do not think this distinction is so cut and dry.
The vast majority of individuals I work with come in with issues they have been struggling with to some extent, the greater part of their life. These issues may take the form of depression, anxiety, feelings of inadequacy, physical symptoms, difficult with anger or other emotions, difficulty with all manner of relationships, sexual dysfunction, identity issues, dissociative symptoms, etc.
In short, the issues they struggle with are often /deeply rooted /in their personality.
These deeply rooted issues or personality traits are not changed easily but with motivation, work and perseverance, they can be changed. The extent to which they can be changed depends on all the variables. In some case the changes might come sooner and to a greater extent and others might come to a far lesser degree and slower.
The variables include the nature of the traits and issues and what role they play in the individual’s personality / psychic structure, the individual’s motivation and perseverance, the variable factors of the individual that make them amenable or resistant to change, the individual’s support system as well as other environmental factors, the goodness of fit between the clinician and the individual, and the therapists skill, etc.