In 2012 un-upvote of a comment was implemented Cancelling upvote on comment?, but with serious restrictions:
Comment upvotes can only be undone for a short time and while you did not navigate away from the page. Once you undo the upvote, you cannot upvote the same comment again.
From Cancelling upvote on comment?
As a reason it was said “We do not want flip flopping, nor do we want people to un-upvote stuff from the past. The only use case is undoing a mis-click.”
It is not clear why “we want” such way. There are multiple comments to the answer and Comment upvote, undo, and then re-upvote: why not?, describing different scenarios that will benefit from giving user ability to upvote and revert when user needs.
I didn’t noticed any comment explaining what harm would be if user changed the vote to the comment more than once.
The answers to questions Why is re-upvoting not allowed for comments? and Comment upvote, undo, and then re-upvote: why not? also do not explain the reason, they refers to original implementation answer and require user to “have to take a very concious decision to un-upvote” (within 1 minute)
I had a few times, that after comment’s upvote, I start to think, was it correct or not, and having time pressure(to decide within 1 minute), I made wrong decision, that later I wasn’t happy about.
Please remove restrictions on un-upvote and re-upvote and allow users to express their opinion any time.
Changing the mind over the time is in a human nature and should not be prohibited.
Update: Thanks to Magisch’s comment for explaining one of the reasons for downvotes. I am aware that comments are considered second class citizens, but they are not temporary. “They don't. They stay around.“
If the comments and comment votes are not important for site, why site developers implemented some complex logic to restrict the votes on comments? Give users more control, in particular if it doesn’t cause any harm.
“Comments are content; votes on comments are content as well.” and if users have an option to upvote the comment, they should have an ability to change the vote, when they change their opinion.