Can cedar siding that has been stained a dark stain be lightened to the point you can change the color back to a natural cedar color?

No, ...it`ll be too thin by then, and you`ll never get the joints. Remember the thick part is at the bottom, but the top of the exposure will thin to the point water will soke both sides. besides,...cedar shingles left untreated without stain, paint,oils, clearcoat, or wax, will discolor in sections,collect mold spores, dry too fast and warp, curl from heat and cold, and just plain look ugly in five years or less.