Is it illegal to convert my car's halogen headlights to LeD headlights?

Not illegal.

Car halogen lamps can be changed to led lights, but it should be noted that the vehicle's original halogen bulb reflector bowl and lens focus compared to the led lamps are not the same, led lamps may be formed by the column of light is not so uniform, according to the level of product quality is different, if the heat dissipation and constant current effect of the led lamps is not good, the lighting effect will be relatively poor, and the light heat dissipation did not Handle well on the vehicle itself is also a great impact.

If you change the led lights without lens is not over the annual review, in the annual review of the light requirements in the "motor vehicle operation safety technical conditions" in chapter 8, section 5 clearly on the headlights near the light should not glare, as long as the bare connection xenon bulbs or LED headlights without a lens are classified as unqualified.

Expanded:

Note:

Consideration of the annual audit: users have to consider is the problem of the annual audit, after the modification of the headlights on the car and the driving license on the photo will not be the same, last year's audit may not be able to pass, the specific can be through to see how the policy of each place.

Some affordable low-cost cars are originally equipped with halogen headlights. Halogen lamps are not bright enough and dim with use. Many users can not wait to change the halogen lamps into hernia lamps or led lamps, remind everyone halogen lamps in the fog driving effect is the best, are changed to foggy day vision is not so good.

Baidu Encyclopedia - LED lights

People's Daily News - about the lights those things: your car headlights bright enough?