It is very possible. Exposure can change a person. There are people who due to their upbringing finds it so difficult to interact with people since they were not allowed to socialize with people from childhood and so even when they go to school they still behave that way. I have also noticed that even though some people have had a lot of exposure, they remain introverted.
Well, I believe it is difficult to categorize people as introverts or extroverts because people behave differently depending on their mood, the people around them, and the nature of the gathering. In a gathering, you may notice a person who appears to be an introvert making the loudest noise and talking the most. Therefore, I think that everyone is a "metrovert," the ability to blend the traits of an extrovert and an introvert.