If you have upvoted an answer post, it is impossible to downvote it after a grace period, unless it gets edited, no matter how ancient the answer is or the upvote is. This turns out to be counter-productive if the answer was correct at the time, but is now incorrect.
For example, a question might ask: "is it possible to do X", and someone might have posted an answer saying "no, it is not possible". At the time, that answer was correct, so I upvoted it. But years have passed, and now it is possible to do X. That answer is now incorrect. How can I downvote it?
(The problem is exacerbated by accepted out-of-date answers pinned to the top. If the OP is no longer active, an out-of-date incorrect answer might be pinned to the top, and it cannot collect as many downvotes as you might expect, because some users are locked out of downvoting it. All of this misleads users into thinking that answer is useful.)