I think it may be possible. I was just talking about this with another INTP. We recall being way more extroverted as children, and taking a glance about those (not very scientific) websites that talk about types in children, I remember being more ENTP than INTP, whereas my friend agreed with the INTP descriptions, only that wasn't so much time locked inside his room.
However, if I think about it deeply, I have never been as extrovert as "real" extroverts. I was the kind that pondered whether to climb a tree or use the swings for several minutes lying on the grass, and preferred (most of the times) check for woodlice or 4-leaf clover than play hide and seek with the neighbours in our dirt street. I attribute it to my parents that never let me go outside and play with others until late. They preferred me inviting others to play to my house. That and a couple handfuls of shitty things that happened to me, which led me to not believe in people and knowing that I'm better off alone. I now really enjoy being alone, and even spending time with friends for a long time is a drag.
So, summing up, I believe that if you experience enough crap in your life, you can change. Besides, it's very different how you experience life as a child and how you experience it as a teen or an adult: you discard some habits, take up anothers, make choices, react unconsciously to what surrounds you.. it's very difficult to find a point in which you can say "I've been like this since forever". At least that's my case. Anyone else?