Sometimes info in a comment causes me to want to retract a downvote on an answer.
It's not possible to change a vote once its locked in unless the answer is edited, but since I can edit answers, its easy enough to work around this by e.g. adding a dummy HTML comment to the answer.
Some day, my dummy workaround comments will conquer SO.
It seems like, given that nothing actually prevents a person who can edit an answer from harmlessly editing the answer to change their vote, that it would make sense to just allow you to change your vote if you have answer editing privileges.