It dosen't actually give much info, the video is just of the detachable ball being fitted/removed. Although it states you can convert an existing bar it dosen't give any more details, I asumme you would need a Witter bar I'nt first place to do this.
I think the receiver / flange part is welded to the cross bar, so at the very least, this would need to be replaced. Have a look at the fitting instructions shown on the site listed by Draiq. Probably cheaper / easier to by and fit a complete towbar and sell the old one on.