I just did this with my sw tank, I went from crushed coral to sand and what I did was drain the water (save it if you can) and placed the fish and coral into the tanks that I drained the water into. Then begin the process of removing all of the "gravel", then add your sand after you rinse it of course. After that you can add your water in, but be careful not to disturb the sand or else you will have a milkshake tank for a few days while it settles. I had a slight ammonia spike for about 45min to an hour then all my levels evened out and went back to normal and haven't had any problems since! Hope this helps!