replacing ceiling mount shower head
Yes, it is a big job.
Your plumber will need to open the wall from the valve that is controlling the rain head to whatever point where you want your new wall mounted shower head. At that point he will need to open up enough of the wall (width of a stud bay)to get a 2x4 in there for blocking.
If you really want to go ahead with it, you could save yourself some $$$$ by opening it up for him. Not sure if I would want to pop tiles for someone... Is there any access to the wall behind the valve?