It's really quite simple. If within the first hour of posting a comment (no point in revising by then) you wish to revise your comment, you can. However, comments which have been edited are clearly, and I cannot stress this enough, marked as edited and all replies are marked as being towards an earlier revision. Perhaps allow only a single edit on the comment. If you're worried about people changing the content of their comment after it's become popular, allow like five letters of difference maximum, enough to correct minor spelling errors (swapping letters, forgot a letter, etc.) This can be implemented simply, but it has to be in a way that doesn't allow for ambiguity on whether or not a comment has been edited.
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