You can definitely plane down your deck boards to get a new surface. I've talked to several owners who use planers in a similar fashion, planing old floorboards or siding boards to get a new surface.
You should not have to do any sanding, but I don't imagine that the planed surface will last any longer than the sanded surface. This is where a good quality deck stain or deck sealer is needed. I have seen several deck stains with a 5 year warrantee on horizontal surfaces.
As far as the knife longevity, planing deck boards will be hard on the knives due to the amount of dirt and particles that get ground into the deck boards from weather and walking on the deck. If the boards are straight and of even thickness then you shouldn't need to take more than a 1/16th off.
There are wood-like deck products that can be screwed and cut just like wood, but have better weather resistance.
Jake
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