The original set-up should be vented. If you plan to add onto the existing drain line to make up the 8 feet, I don't see a problem with just making the addition. You could always make a transition to PVC and add a tee with piping extending up above the top of the washer and add an Air Admittance Valve for venting. Not all areas allow AAV's but if your current system is not vented, I see no difference. At least the trap will not be sucked out when the washer discharges.