to continue on liamson's point, a true extrovert gains energy from social interaction. therefore, it's actually a stress release and recharging event to be around other people.
and it's very common for people to slide over in all of the measurable aspects of personality.
for example, when i was about 20 and tested with the meyers brigg (personality test) i was a fairly severe ENFP on all qualities. i definitely thrived and preferred social contact, and my life was a reflection of that. and then i grew up and a lot of that changed.
i retested at the age of 32, and i was still on the ENFP side, but would be described as mild extrovert, and far more central in the other qualities as well.
i'm fairly convinced that if i tested today that i would be probably close to the middle. my personal belief is that as you mature and grow, (especially as a Christian) you become more tolerant and accepting of vocational/environmental limitations and challenges which, like any muscle, will create strength where there is weakness. isn't that cool that we have the capacity to evolve in so many ways?
living alone has definitely made me far more introverted, however, i would go bat crazy without a fair amount of contact and my animals.
so yes, people do change, and it would be unusual if they don't shift at least somewhat. i also know a couple painfully shy introverts who have slid way over to the more neutral position.