Do it slowly over time. I would change out about 1/3 at a time to give the bacteria and pods
already established in the sand time to migrate over to the new sand a little at a time.
I generally think you might have an NO3
spike when you do it, b/c you are going to stir up allot of the stuff that sits on the sand into the water column, but do it with PWC
's and you should be fine.
To add sand while the tank is filled, I have heard of people using a 2"PVC
pipe to pur the sand in and get it where they want it, or just filling up a small bowl and sinking it slowly in the water. Be prepared for a sandstorm, but it shouldn't hurt your fish too much since in the wild, they have to deal with it when the currents get wild as well.
DO you have fish in there yet? If not, you can just bite the bullet and change all out now and then watch your water parameters to see if you go through another cycle.