propane question (RV type)

[upnover]

I was given 3 40 lbs RV type tanks today. Same as a BBQ tank, just taller. The problem is they are the old style, and can't be legally filled. They have to have the new type valves installed. My question is what's the difference in the valves for a 20, 30, and 40 pound bottles? I was thinking about taking the valves from a newer 20lb bottle and put into the 40lb bottle. But when searching Ebay, I found the new valves listed for each of the sizes.
Thanks

Also got a natural gas bottle too, it looks similar to a Welding type gas bottle