loader.loadModel() does everything that loader.loadModelCopy() used to do. You can use loader.loadModel() for all your loading needs.
For optimizing your scene, there have been many posts on this topic throughout the forums. Short answer is do table.flattenMedium() or table.flattenStrong() after you have loaded everything and put all your objects on the table.
To change scenes, just remove the old scene, with something like table.detach() or table.removeNode(), and create the new scene.
David