I just bought a set of Hella 500s off eBay, but forgot to confirm that they were fogs rather than driving lights. If I wanted to convert them to a fog pattern, do I need to change the reflector? The lens? Both? Is it even worth switching them to fogs? I already have driving lights, so I'd prefer to have fogs as well.

Well, you'd have to buy a replacement lens/reflector. If you look, the lens is bonded to the reflector making it a complete unit.

Cheapest route, buy a new foglamp kit from rallylights.com. You can buy the lens/reflector assemblys separately, but it actually ends up being more expensive than picking up the entire kit.

[then find a way to mount both sets of lamps on your car] ;)

ditto

If you bought them from a trader, can you return them?
If the seller is private it's probably easier to re-sell them and get the lights you want elsewhere.
Changing them is not really an option worth considering.

John

what they said :)