There is more than enough bacteria in your filter to handle a substrate change. Do large vacuums of the gravel before you make the switch. Dont wash filter material before the switch either. Wash your sand (I say again, wash it). Remove gravel, and put the sand in slowly. Can leave the fish in the tank, can lower the water levels during water change, can leave water levels intact. Your preference. If you're going to do it, do it all at once.
can also add sand on top of current gravel, just do large vacuums as the sand will sink to the bottom in time and trap whatever gunk you have underneath there currently with negative results (gas buildup, etc)