What Eioru means is, if someone edit his posts in a thread instead of posting a new reply, could the whole thread be tagged as "new"? I wish the admin can implement this feature too, but seriously doubt it...
If the post you want to edit happens to be the newest reply in a thread, you can work around by copying the exact content plus your editing onto the clipboard, then delete that whole post, and repost the new content by pasting from the clipboard. If it's not, then the best you can do is to edit it, and then post a short message notifying that you have edited an earlier post in the thread...