Are you replacing this with a similar sized fiberglass unit or plan to tile it in? If this unit was installed when the building was constructed, it may not come out in one usable piece, without removing part of the wall/doorway. What type of trim surrounds the unit right now? What type of tools do you have to work on this project? I'll give you the steps, once I know what we are dealing with.

Your plumber is correct. The edge of the unit was attached to studs and then covered up with drywall and tile. Is your door opening large enough to remove the shower unit in one piece? Demolition can be challenging if you've had no experience, as there could be water, gas and electrical lines behind the drywall. Without knowing, I'd be very careful in cutting out the drywall. You could either score the drywall at the next stud and then have fun with a hammer or actually cut it through. Use a stud finder to find the next stud and then draw a vertical line at the center of the stud. Once the drywall is removed, you can see whether the shower unit was attached with screws or nails. Realize that you may have to move the drain and/or faucets for the new tub to fit. If you are still up to it, let me know, and I'll give you step by step instructions.

Fiberglass is very easy to cut with a sawsall or jigsaw, though make sure that you wear a mask so you don't breathe in the fibers. I'd keep the door closed and then vacuum carefully after everything has settled, if you need to cut it. If it is just a bit larger than the door, you could remove the door frame and then take it out that way. Here are the steps to remove your shower: Turn off the water supply to the shower and take down and remove the existing shower doors. Remove the knobs and decorative trim for the existing shower fixtures. This usually involves lifting a decorative cap from the knob to reveal the screw holding the knob on or using an allen wrench. Detach the showerhead pipe by gripping it near the wall and turning in a counterclockwise direction. Remove the tub strainer by inserting pliers into the crosspieces and turning in a counterclockwise direction. Underneath the strainer is a threaded pipe called a drain shoe that connects to the drainpipe in the floor. Remove the shoe by turning it counterclockwise. Remove any existing tiles or drywall to expose the stud wall, and then use a pry bar to remove the wall panels. If screwed rather than nailed, remove the screws first. Pry up and remove the existing shower pan. Let me know if you run into any problems and what the results are.

Let me know how it works out. Don't get frustrated if you run across a challenge. Get back to me before you decide to do anything extreme. You may find that adding a length of pipe to the wrench you are using to unscrew the drain will give you more torque and make the job easier.