Question about downloading scenery

When you download scenery for FS2002, it appears some of the downloads are replacing your defaults, and some state that certain ones don't affect your defaults. How do I know?
Also, if a scenery file does replace the default file, can you get back to default in all cases, if you so choose?
Any help is appreciated.
Also, if a scenery file does replace the default file, can you get back to default in all cases, if you so choose?
Any help is appreciated.
