Until now, there was no way to edit a video that was uploaded to YouTube, without creating a new video file and URL. Yes, YouTube introduced an online video editor a while ago that lets users re-mix and mash-up their own and creative-commons videos. But all videos created in that editor got a brand new video page and URL.
But in fall 2011, YouTube introduced a new type of video editor that lets you make changes to videos on your account without changing the video URL. This is a great feature because you can update videos without having to worry about updating shared or embedded links.