What I did when I changed my 55 gal from black gravel to sand: Bought pool filter sand, and bought a trashcan (like one you put in your kitchen that can hold water) then poured the bag of sand into the trashcan and stuck a water hose in it. fill the trashcan up with water and allow it to overflow for 10-30 minutes. Do this until you can stir it and the water stays clear. Then I bought a cat litter scoup and scouped out the gravel from the tank (after doing a major gravel siphon, so no dormant waste would come up). After I removed a good amount, I simply grabbed a cup and used it to shovel sand from the trashcan into the tank. I did all of this while the fish were in it, they stayed on one side and i poured it gently on the other. I never had any complications with water quality, and the fish's health never changed.
Benefits to gravel: any color you want, fun to watch fish pick up/move gravel, easy to siphon
Cons to gravel: waste falls throughout each rock, not a "natural" look
Benefits to sand: Waste mainly sets directly on top of the sand, so there isn't much trouble siphoning up the waste, looks much more natural, it's AWESOME when fish swim around, kicking sand with their tail (if falls back down quickly) and its MUCH cheaper ($8.00 for a 50lb bag of filter sand)
Cons to sand: difficult to siphon if you're used to siphoning gravel, but it's not all that hard, just don't siphon a lot at a time.. really thats the only con i can think of.
I have an aquarium with sand, an aquarium with actual lake bottom ( boiled it for hours, then used it.. it's mainly really fine grained sand), and a pool filter sand bottom aquarium. I love all of them, but the pool filter one is the easiest to clean. The gravel sucks to clean only because every section has to be siphoned and the lake gravel sucks because it's so fine that it gets siphoned almost immediately through the tubing. After getting used to the pool sand, it is by far my favorite substrate.
As for the mixing, i intentionally left some black gravel in, just cuz it looks neat lol.