Changing light bulbs does not void your warranty.
However, avoid the common mistake of buying a bulb that is not rated correctly for your car. Your brighter lights run the risk of melting the wiring harnesses, which is not covered under warranty because you broke the car by replacing a part with something that was not rated correctly. It's the same difference as putting a 100watt bulb in a lamp rated for a 20w. When it melts, you can't blame the lamp maker. I know at least half a dozen people who've done this (this impacts halogen bulbs, not HID's which are a different beast)
Current: '15 M235i Vert
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