I have one of those one piece molded fiberglass tub/shower enclosures with a Symmons Temptrol valve, tub spout, and shower. I assume the valve is original from 1985. The shower has a Delta removable hand held shower head. A couple of years ago the diverter mechanism in the valve quit working. I had a plumber replace it, but the result is not great. In any case I dislike the one-knob control of temperature and pressure. The shower head is also shot and needs replacing.

I would like to replace the whole mechanism with something that has separate controls for temperature and pressure, and with the height of the shower head adjustable. Also a better quality shower head than the kind I have. (E.g. not plastic.)

Is it possible to accomplish this without messing up the tub/shower enclosure? If so, can it be done working through the existing openings? (If need be the back side of the wall containing the valve is accessible from the next room, but of course I'd rather not tear it up unless necessary.)

Could this be done entirely by a plumber, or would I need some other sort of contractor as well? (I do some of my own home fix work, but not plumbing. I could do the wall repair.)