I think there is a thread somewhere on the forum about adding sand to an established tank. Try the search option.
I replaced the gravel in my 180 gallon tank with pool filter sand without removing any of the fish. It really isn't hard at all. I just removed the gravel and removed water like I was doing a huge water change. I added the sand to the tank (without any filters running) and then refilled. I ran a powerhead with a quick filter attachment for a bit to pick up any of the loose debris from the sand in the water column, then turned on the filters to the tank.