James Delingpole's post today about "trolls" mentioned Paul Johnson, a great British historian who moved from left to right. I can think of many other examples: James Burnham and Frank Meyer, both foundational in the creation of National Review, Whitaker Chambers, author of Witness, P. J. O' Rourke, Sidney Hook, Irving Kristol, Norman Podhoretz, Malcolm Muggeridge, Melanie Phillips, David Horowitz, and many more.
On the other hand, it is hard for me to identify many who moved the opposite direction: Garry Wills, David Brooks and David Frum (though they really moved from right to center), David Brock (whom no one should count as an intellectual--he's a hired gun with no principles).
Who have I missed?
Why to they do it? I'm sure there are a variety of reasons that people move from the left to the right, but I feel the central elements are (1) a realization that the left is ultimately the enemy of freedom and economic vitality and (2) the perception that the conservatism (or classical liberalism, if you prefer) is based on universal first principles.