Do people really change what? Change their personality? Or change their overt, outward behavior? There's a subtle difference.
While overt, outward behavior can change, personality is stable and fixed in childhood. As Phat said, a narcissist will always think highly of themselves and an introvert will always enjoy being alone. An introvert can learn to be outgoing, but the inner tendency or desire will still be towards introversion. To prove this, look at longitudinal studies of personality. There are a lot of studies available to back this up.