As I noted elsewhere, I think penalties, even fairly harsh, for changing votes are reasonable given the inherently sequential nature of voting. Your vote may induce others to vote after you and then when you change your vote, their votes may be hurt.

Changing is needed in some edge cases like when a post is discovered as plagiarism after people already voted on it (actually even this is debatable since the early voters could and perhaps should have been more careful in vetting, and the solution is just pile on downvotes) but it shouldn't really be normalized to too large an extent.