If the old receptacle is in the middle or beginning of the circuit, reconnect the hot, neutral and ground wires with wire nuts and push them to the back of the electrical box.
250.146(D) makes provision for the isolated ground conductor, but isn't there another code which would require those "old" EGCs to be bonded to the box as well, along with the bare EGC of the new 3+Gnd cable?
