This is silly: I can edit the answer to allow myself to change my vote, so what is the point of locking it? Basically, vote locking shouldn't apply to people with enough reputation to edit other people's answers.
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Since you have to edit the answer in order to cancel your vote, there will be an appropriate entry in the post's history. For instance, this prevents users from "strategically" downvoting answers then lifting their downvotes later without anyone being the wiser.