Reverse ball arm adaptors????

I recently bought a 75 year old house. The previous owner replaced the ball arm with a regular one for the shower head. I located the old shower head (which matches the rest of the fixtures in the bath) in the cellar. I have been unable to get the arm off to replace it with a ball arm so I can put the origional shower head back on. Is there any type of "reverse" ball arm adaptor so that I can put the old shower head back on?