You should be able to place pavers over existing concrete, assuming it is in fairly good shape and the patio won't be too high with both a sand base and the new pavers.

Try using a grinder with a 7.5" diamond blade 120 grit before you actually put the paver over the concrete. It will allow you to smooth down all the high areas and level out the existing concrete. Be sure to use a multi-purpose thin set with a bonder in it for extra added strength. If the pavers have a sealer on them, be sure to break the seal using a TSP to wash down the surface.