Hello, welcome onto the board.
There are certainly plenty of cases where, from the factory, the only difference between the halogen and HID or LED versions of a given vehicle model's headlamp are exactly as you say: one has a halogen or bi-halogen projector, and the other has a Xenon/bi-Xenon or LED/bi-LED projector (plus ballast/driver as applicable). If you have a halogen projector headlamp, and you can remove the projector out the back without breaking open the lens, and you can directly bolt in a legitimate HID or LED projector and button everything back up so it's all appropriately sealed, then it's a really hard stretch to make a technical or legal argument against it.
The devil is in the details, though. The projector has to be a legitimate one, not a lookalike knockoff. And you have to be dilligent in checking to make sure it's really fully compatible with the housing -- as mentioned in the
What's Wrong With Projector Retrofits post, sometimes a projector that will physically fit won't be optically compatible because you'll get stray light bouncing around in ways that cause upward stray light and/or glare. This isn't really hard to check for; if you have a direct bolt-in projector situation, bolt it in and light up the lamp in a dark room with light walls and ceiling, point the headlamp at a wall, and look above the beam pattern (on the wall and on the ceiling) for streaks or spots of stray light not present with the original halogen projector.
Another potential "gotcha" is that the HID or LED projector might not mount quite exactly the same as the halogen projector relative to the axis of the lamp housing. Not a major issue if it's a bi-Xenon or b-LED projector, but if it's a single beam projector producing only a low beam, then it might not be possible to aim the lamp correctly (on low beam) and still have the high beam aligned where it's supposed to be. This can roughly be checked visually in the same darkened room.
