This question already has an answer here:
I have enough reputation to upvote but not to downvote. I upvote a post. The post is then edited (or very little time passes). I then change my mind and attempt to downvote it. I am told I cannot due to my low reputation; and my upvote stays in place. I think my upvote should be canceled when I attempt to downvote, since clearly my intention is to no longer upvote.
Update: See blahdiblah's answer and his own comments thereon: they explain what I meant better than I did.
Question 8390 is similar in wording but requests that every voter need to click twice to switch votes (which I don't agree with). My request applies only to those with the upvote but without the downvote privilege.