The funniest thing is that personality is actually really stable and temperament is fairly fixed, and in most cases, absent major trauma or brain injury, when adults think they are "completely different people" from who they were earlier in adulthood, they haven't really changed on a fundamental level; they are still completely recognizable as themselves to others who haven't interacted with them recently. Rather, what changes, with some effort, is typically relatively superficial behaviors and attitudes. Because personality is so stable, it takes so much effort to achieve even relatively small shifts in perspective, behavior, or attitude, and so that expenditure of effort causes the person who has made such a change to overestimate the degree of difference from their former self.
Great I'll always be that creepy introvert
Introversion/extraversion tends to "wander" more than other traits, but generally if you start out an introvert, yes, you will always be an introvert. You may become a more social introvert, and you may gain social coping skills that make you a popular and fun introvert, but yeah, basically you will probably always be an introvert.
It is, however, important to not confuse "social skills" with "personality". The former are learnable and increase with experience, the latter is a product of the interaction of your temperament with your (mostly) early development and is usually pretty stable.
That said, there are some trajectories that are generally pretty predictable as people age, like becoming less emotionally labile (thanks, calcification!) and better able to regulate their reactions.