With the sand, all the critters that live in it live at certain depths. In my case, I had a deep sand bed, so I had a lot of "layers" of different life. When you disturb 200lbs of sand, take it out of the tank, put into buckets, and put back in the tank, all those critters that lived at certain levels of the sand get all mixed up, and the ones that lived near the top get crushed and killed in the process, causing things like ammonia spikes which SPS corals really do not like. On top of the die off from microfauna and bacteria you have all the built up detritus in the sand that you let loose into the water.
So... to say it short, even if you only have a few corals, the price of dry sand is so cheap there is no reason not to replace it compared to the cost of life/corals in the tank. You could go the cheap route and go with the Home Depot pool filter sand (silica free type) for like $5 a bag. Any dead/dry sand will become live sand in a short time if placed with live rock in a tank and going from dead to live doesn't cause any real issues as long as you pre-rinse it, short of reduced bio filtration, compared to the issues caused by going live to dead to live again.
One note on adding sand to an established tank.. always add small layers, no more than 1/2" at a time, this allows all the critters that live at certain depths to move upwards accordingly without getting crushed, which would happen if you add too much at once.