Whatever you do, be careful when trying to loosen the ball assembly. If
its hard to loosen, DO NOT force it. You will end up twisting the
entire shower body inside the wall and damage it. Thats what I did.
Fortunately it did not break or leak, but my faucet is a little crooked
now.
Make sure when you shut off the water to open the faucet to relieve any
pressure.
If the ball assembly cannot come off, you might want to use a small
hack saw and carefully saw on 2 opposite sides of the ball carefully
without hitting the threads of the shower body, then gently pry off the
ball. Of course you will have to by a new ball assembly, but its better
than replacing the shower body.