The sand was used to protect the bottom of the pool - as you probably already know. There is no other reason for it. I use sand instead of topsoil in my garden, but that's me. Some disadvantages to sand are that it holds no fertility or moisture. If you put topsoil on top, when the roots grow down into the sand, there will be very little to keep them there. Not sure how that would pan out. I guess I'd be inclined to pull out the sand and replace with topsoil.

The new surface should crown a little on top to allow for settling. Otherwise in 3 years when the new topsoil has settled, you'll have a swamp.

If it were me, I would spread topsoil over the top of the sand, then use a garden rake to incorporate the sand and topsoil together. And as dchall wrote, crown the surface for future settling.

Thanks for sharing, I was curious about the sand. I took out an 18' round pool, after 2 yrs of sitting and chemicals depleted I tilled the sand today and will incorporate top soil, retill and place more soil on top. I am making a medicine wheel for my herbs and vegetables.