Good article on automotive lighting. It has come along way in recent years.
Aftermarket HID kits are ok for low lying fog lights but otherwise they simply focus there beam to high and end up blinding anyone in there range.
These powerful beams need levelers or at least directional guides on the aftermarket kits.
I think that depends heavily on the quality of the product and the quality of the installation.
The problem occurs when people buy insanely bright HID bulbs and ballasts from China on eBay and plug them into a reflector-type enclosure meant for halogen bulbs. There is no cutoff and the light splashes everywhere, unless they're aimed almost directly at the road in front of the car. There is a tendency for people who are upgrading their lights to feel that in order to see an improvement they have to aim their lights higher so they see further down the road. This is the dangerous part, because they're so high that the lowbeams act like a highbeam and consequently blind oncoming traffic. That's the opposite of safe - sure you can see where you're going, but now all the oncoming traffic is blind and more likely to cross over the center line and cause a head-on collision.
In contrast I have seen some very nice projector-style HID retrofits where the whole assembly was changed for a projector-style beam with a sharp cutoff that will not blind other drivers. The lights are adjustable and, assuming it is a quality installation, the lights are aimed properly from the beginning. You can achieve the same lighting intensity, in some cases better, than a factory installation without offending other drivers. This is much
more expensive, so it is less frequently used.
My question is - if our vehicle already has projector-style headlights, can I just swap out bulbs or do I need to replace the whole unit?