I face the same problem raised in this question: Vote too old to be changed, unless post is edited “problem”. I upvoted an answer which I now think shouldn't be promoted as helpful. I'd like to reverse the suggestion I provided via the up-vote.
If I am correct about the history behind this change, it was implemented to hinder those who game the system by down-voting competing answers.
What I would ask is the ability to down-vote the same answer at the cost of my rep so I can neutralise my up-vote. The user who posted the answer can keep the cumulative upvote+downvote rep. If there are concerns about people using this to game the system, can at least upvotes be neutralized by a corresponding down-vote if not the other way around?
To reiterate, I'd like the ability to neutralize any voting action I perform, not revert it.
In this context, I don't think suggesting that I not vote for an answer till I'm definitely sure it's correct, is very helpful. It'll mean that only the cocksure cast their votes, or that most questions go without any votes to distinguish between answers.