1) I vote up a comment. 2) Later on, the author of the comment edits its contents. 3) After such change, I don't consider the comment valuable as before anymore.

Shouldn't I be able to vote it down? In reality, I am asking if the positive/negative votes for a comment should be rolled back when the comment is edited. It makes sense to me that users vote for a comment in particular, not the future versions of it.