Laying pavers over existing concrete patio and extending to compacted gravel base
I know it seems excessive but I would HIGHLY recommend removing the concrete. The reason is that the pavers over the concrete will not settle at all while there will be some settling of the pavers over the compacted ABC. You will end up with a very ridged area and the point where a paver sits partially on the concrete and partially off will become an ugly not to mention hazardous edge. 

Body of Knowledge:
5+ years laying pavers, rock walls and sandstone patios.