How malleable are personality traits, if at all?

Under the right inputs opinions and worldviews can be changed relatively easily, but what about the more subtle stuff underneath? Can a workaholic with a strong drive later in life become lethargic? Can an innately introverted person later in life become innately extroverted?

Those aren't the best examples, but that said my intuition would be that these tendencies are too deep rooted to be significantly altered and can only be superficially mitigated against one way or another.

If this is to a degree incorrect I'd love to hear some anecdotes.