I think BuiltWith will do this for you. Plug in the domain and it will tell you which technologies it is built on. Most of the well-known video sites are custom jobs, but I'm sure WP could do what you want.
Maybe the prudent approach is to create a parallel system which includes all the newer technologies. Instead of an all or nothing transition, transfer one (or a few) functions at a time. If something goes wrong, you have a failback. If things work well, you can speed up the transition until ev...