Re: diverter conversion?
Two options I can think of off the top of my head. One is to locate a single handle valve that has an escutcheon plate large enough to cover the existing holes. Second is to replace the wet wall formica.
As misfitter suggests, you'll need access to the plumbing to change out the valve, the best way to do that is through the tub side, unless you've got a closet on the other side of the wet wall that will give you access.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!