When Tile is put on a wall, it essentialy bonds to the wall to the point where it is really one piece of material. In otherwords, it ain't comin' off. The only reasonable way I have found to do this is to pull the whole thing off, tile, gyp board and all. Besides, there's a 90% chance the tile grout leaked anyway and you'll have mildew and rot in the wall that needs to be addressed. No time like now to take care of it.
Afterwards, just put up some new greenboard, do a basic mud and tape job, and install your surround. If you are replacing the whole tub-shower unit, you can mount the surround directly to the studs and skip the drywall, except above the surround. This would allow you to replace the showerpan, which could probably stand to be done as well. If not, be sure to protect it from falling tile and drywall when you do the demo.
After you're done, use Gel Gloss to restore the fiberglass showerpan.