If I change my mind about upvoting a comment, is there any way to cancel my upvote there?
There's a meta stackexchange thread on this topic, here: Cancelling upvote on comment?. In brief, you can change your mind -- "upvoting" again undoes the upvote -- provided that you do it quickly (within 60 seconds) and without navigating away from the page. Otherwise, no, you can't take it back as far as I'm aware.
It would be nice to not have to decide quickly, but I think the SE philosophy is that comments are not supposed to be important and should be used only for improving posts; thus upvoting comments is considered also low on the significance scale. (In practice at MO, it feels a little different to me, but there you have it.)