I have done that! What I did was get a bucket, and a net (with decent sized holes) and sift all the gravel out. It depends what size tank, but I sifted all the gravel out, and then swished it in the water. All the sand stayed in, and the gravel went in my bucket. Then just water change, water change water change, and more water changes, and the sand SHOULD be out. Then wash the gravel, and put it back in, and it was fine. I always cap sand now, instead of cap gravel with sand. The gravel (other large substrate) seems to hold down the sand, and anchors wonderfully. I did this with my 55, and couldn't have been happier.
Hope it helps,