@Sandy okay there's no way to move a comment from one card to another but if you right-click the timestamp on a comment you can copy a link to it, which you can then paste into a comment on another card:
The advantage of this rather than just copying and pasting the content is that you can see the original context/location of the comment, the downside is that you can't see the actual content of the comment.
Other than that, you can just copy the text and paste it into a comment on the other card :)
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