I wouldn't say it was an instantaneous, wholesale conversion from one philosophy to another but I was definitely changed over the course of a few weeks when I read The Sane Society by Erich Fromm along with a wide selection of works by Karl Marx and Trotsky. I never embraced far left ideals (I've yet to see a satisfactory argument addressing how human nature can be grafted into such a system) but it was one of the first times in my life when vague, unsettling notions I had about society were validated and explained by way of a systemic political philosophy. It was incredibly exhilarating to realize that the discontent I felt was not due to weakness or immaturity but because I was a human being who had correctly intuited the existence of massive flaws in the ethos around me. Or to put it in teenage layman terms: "holy shit, I knew I wasn't wrong about this and now I know why!"
I guess that's more of an awakening / validation-of-intuition than an outright conversion, really. But my point is that the experience was an impetus in shaping me into a more confident person because it showed me that my instincts were for more reliable than I could have ever imagined. Had it not been for that, I may have just accepted I was being a "typical teen full of angst" and then spent the rest of my life trying to grow out of it and be "normal".