Yes, if you want to lay flagstone using sand, you could certainly raise everything up by covering the soil and concrete with sand. However, one other option to consider is utilizing the concrete that's already there, and pouring more concrete over the soil to match the existing concrete's height. Then use mortar to lay the flagstone instead of sand. This is more difficult to do, but is maintenance free longer term.

Here's a few flagstone patio examples in case it helps...

Bryan

Here is a link that might be useful: Flagstone Patio Ideas