I would always replace the entire unit for LED. With HID you can sometimes get away with just replacing the bulbs. It depends on the shape of the reflector and the front lens. They work very well on my 1977 Porsche 911, with very good definition, spread and no stray dazzle. These were quite expensive conversion units from a German supplier (about £120 for the pair including ballast/starters and relays). You can buy the bulbs on their own at around £40 each, which I had to as one had a failure of the dip beam solenoid after a couple of years. My MOT man commented that they were one of the few HID conversions which he was happy about passing. I have Ohlins self levelling struts at the back of my 911. which makes the headlights legal without a levelling device on the lights.
Wilson
