Once you see the difference just the H7 being replaced makes, you'll realize why you don't need to replace the high beam. Most people use the high beam for flashing people, and you can't do that with a HID setup. It would mess up the electronics pretty bad! Plus, if you take a gander at your stock setup, you'll notice that the high's only put out a tad more light after the cutoff of your projector! You mostly notice that the stock output is only brightened by the H4 bulb. So no, it's not necessary to replace it with HID at all. You could do it, but then you'd loose the ability to flash people, and it wouldn't increase your output all that much...So the general consensus is to replace the H7 w/ a HID bulb only... there is no reason to do both the H4 and H7 then?