If you or someone you know has a digital camera, it would be best to take a few pictures and post them on a web site like Photobucket so that we could actually see the situation. It's hard to know if a job will be easy or hard unless you see it.
I'm not sure if you have the large fiberglass single piece tub and shower, but my understanding is that some of those things are so large that you really can't "replace" them because you can only get it into the house while the house is being built and the walls aren't in the way yet. You can always get a conventional enameled steel tub into the house, tho. So, if yours is one of those really big fiberglass ones, then you might have to replace it with an enameled steel bathtub rather than another "all-in-one".