The original bulb + reflector + vehicle was an approved system based on the OEM or lighting supplier performing the FMVSS required testing for illumination/ color/ beam pattern. Once a different type of bulb is installed, that original testing/ certification is not valid. One could take an aftermarket bulb kit, install it, and then perform the required testing. If it passed, then that would be a legal bulb + reflector + vehicle combination. Until then, it is not. As it is, the system would be the new HID bulb and reflector wherein the reflector happens to be the same as the stock one.
Poor analogy: A taillight uses a red bulb, the bulb is DOT approved as a red tail light bulb. I mount that bulb in my front turn signal/ park lamp. That is not an FMVSS compliant installation.