Glass headlights get milky?? Anyway, yes you can't change illumination type legally as the reflector is designed to work with certain bulbs.
Polycarbonate ones do, which is what all modern cars use instead of glass. Polycarbonate suffers from UV and exposure to the atmosphere, eventually becoming opaque unless protected. They are protected as they leave the factory, but over the years the film wears off. Yes, they can be restored with wet and dry, but due to the lack of the protective coating they will much more quickly become opaque again than when the left the factory. My lamps tended to show deterioration in just weeks after being restored.
You can buy headlight restoration kits which include a specialised spray coating supposed to offer the protection, but I have found the clear lacquer, as applied to car bodies after the colour paint has been applied, is as good or almost as good. As a test, I polished up a really opaque scrap lens 6 months ago, sprayed it with clear lacquer, then left it out in the garden facing the full sun. Its as clear now as when I sprayed it.
The abrasion needs to be very thorough and quite bold, using course wet and dry to get completely down to the polycarbonate over the entire lens surface, otherwise the lacquer might react to the original coating. You then work down to finer and finer wet and dry grades, finishing off with Brasso or metal polish. A good wash, thorough drying, they finally the clear lacquer.