It is doubtful you could replace the tub without ruining the surround. As long as the finish isn't compromised on a cast iron tub, it should last a very long time...decades. To keep the finish intact, you don't want to use abrasive cleaners, not drop hard objects that will chip it, and try to keep sand, etc. out of it that could wear away the surface gloss. Even if it is no longer glossy, as long as the iron base is not exposed, it shouldn't rust.