A common bathroom remodel is taking out the old one piece plastic molded shower tub combo and making it a larger walk in shower.
Where do you start?
You have to imagine how did they get that into the room? The one piece units are brought in during house framing so to get them back out they have to be cut in pieces and taken out. This does take the space back down to exposed studs and voila! plumbing replacements can occur!
That being said your best bet for NEW material is a decision between a cast shower pan, acrylic pan with three-piece panels seamed together OR a custom tiled shower to be able to manage the install in the remodel space.