I'm just learning how to work with terrain elements myself and I realize this is a fairly old thread already. But I'm curious, could you save your original file (as you have done) and then copy and paste the same data to a new file, make your changes to the new file, and use reference layers to superimpose the changes between the original terrain and the new/modified terrain? Kind of like a reno, where you'd show as-built and new construction? Maybe terrain doesn't work the same way?