I've never tried sand. It's beautiful, but more likely to trap waste and produce swamp gas, from what I understand.
Cloudiness is usually algal or bacterial and also usually harmless. Changing substrates could affect cloudiness in many ways. If your gravel is caked full of waste, removing it will reduce the nitrogenous wastes in your tank and, hopefully, starve the microorganisms that cloud the water. Or, adding new substrate could loose particulates, but that usually clears up on its own within an hour or so.
Warning: The act of removing old, filthy gravel can potentially release toxic gases that may harm the tank's inhabitants. If you regularly vacuum your gravel all the way down to the bottom, it should be fine. But if you use under gravel filtration, or don't stir your gravel often, you'll want to temporarily move the fish during this activity.