The way this procedure works, if it's going to work, is that you have a brand new test game in Sunset Valley. Save and then quit the game entirely. Put a copy of the nhd file for Everything Island from your previously played and progressed game into the save folder, so that it is now alongside of the existing Sunset Valley nhd file.
Close all windows and start up the game, if you can. Grab some Sunset Valley sims and see if you can travel them to Everything Island and, if so, if the already played and progressed instance of that world loads. Of course this assumes that the underlying .world is now properly installed again.
The serious error has occurred thing usually means that one or more files critical to the save are open or being seen as in use at the time that the game is trying to start up. Just having the folders they are in open in Windows Explorer can do it, or it might mean a file (the nhd we suspect in this case) is too damaged to load.
