If you CLEAN the old side you can get it to where it ALMOST looks new. Then you can stain it and have it come pretty close to looking the same. Pressure washing is the ticket here, but, you have to be VERY careful not to gouge the wood with a PSI that is too high. I rarely go over 2500 PSI on decks. After it dries, it will likely be fuzzy from the cleaning. Gently sand it with 150 or 220 sandpaper to remove the fuzz.
P.S. You don't necessarily have to pressure wash. There are cleaners that you can spray on the old side, let it marinate, then use a stiff brush to scrub the board, rinse, and let dry.
Yes, I agree cleaning the old side will help to look almost similar to new side. Pressure washing is a good suggestion, but that should be done with the help of professionals I had good experience with deck contractor Fairfax, VA. When I remodeled my house in last winters.
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