LOL. FR already HAS an edit function. It's called "Preview". I know, I know, nobody reads their own comment before hitting "Post" any more than they read the OP article. It's a matter of tradition....
But if you want an "edit/correct prior comment" feature, code it up and send the source to John Robinson and see what he says. :-)
Of course the problem is more one of maintaining database transaction consistency, than of user interface. So if I were specifying such a feature, I would keep it real simple:
How dare you suggest that I should proofread my own posts! lol.
Merry Christmas!