The stall probably stops a foot or so from the ceiling, I'm guessing?
Cut the drywall about an inch or two above the fiberglass (there's a lip behind the drywall that you can't see) and down the sides. You'll have to use a reciprocating saw to cut the shower into three sections, but be careful: sometimes the plumbers change the valve to the side of the shower opposite where the plumbing comes out of the floor, and they may have run the pipes in the void between the fiberglass floor and the subfloor. Found that out for myself, luckily I didn't cut a water line.
The shower should come out pretty easily after it's cut away, although you might have to remove a few screws. Once you get that far, we can give you more advice about where to go from there.
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