If you have edit privileges, which you do, then a fake edit to unlock votes seems like the way to go if you truly regret the downvote.

If you have edit privileges, which you do, then a fake edit to unlock votes seems like the way to go if you truly regret the downvote.

If you don't have edit privileges, then you can write a comment requesting the OP to edit the question "because I downvoted by mistake / I fatfingered the downvote button while trying to upvote / I deeply regret my downvote". If the OP cares, the OP will edit the post; win-win. If the OP doesn't care, then I guess that's a reason not to lose any sleep over this.