By extraversion I mean the Big Five extraversion which is broader than the popular understanding of the world, since it also includes stuff like assertiveness, activity level and excitement seeking and not just sociability.

I mean an actual change in the trait itself and not just behavior. Someone who behaves in a less extroverted way due to having more family- or work-related responsibilities and doesn't have time for socializing but still feels the same urge to be social doesn't count here.

Hhhmmm... I experienced the opposite. As a 14-16 year old (8th to 10th grade), I ran with my expanding group of friends especially my core group of close friends but I still had a fair amount of left over childhood self absorbtion. It was a balancing act. In addition, I was living at home, with all of the repression associated with that.

By the time I was 25-30 years old (residency, marriage and early practice) I had developed my personality fully (which was significantly more social) and had my spouse matched my social interest, I'd have been even more "out there" than I actually ended up being.