as long as the old slabs are sound and not wobbly then there's no reason why you can't bed the new flags on top of the old ones, but a few things to consider, firstly you need the surface of the new slabs to be 150mm below the damp course of the house and secondly the new slabs need bedding on mortar with five dots ( one on each corner and one in the middle ) not just dry sand/cement....the last point to think about is the exposed edge......is there a wall along the edge of the existing slabs at the drop to the lawn?