Have you ever tried to reverse back your vote on answers, that you already up/downvoted some time ago? and you get messages like, "You last voted for this answer.... and your vote is now locked..." ? if you were like Oh No you didn't!! then keep reading..
Apparently, if you really want to upvote/downvote for whatever unknown reasons you have, you can still do it, and all you have to do, is just make any edit to the answer yourself..
If you see the below, image, I tried to downvote an answer which I had already upvoted 2 months ago, and it basically told me to screw myself over, but after editing the answer, I was able to bypass the system restriction.
This obviously does not make much sense, but I give two hypotheses in my answer below anyway.