If you go with a plug and play HID kit, like the Xenon Depot kits, you can always remove the kit and revert back to stock if a problem arises. If you do HID retrofit, then the headlight housings wouldn't be warranted, since it's been modified.
If you decide to change your bulbs, I recommend against going with higher wattage bulbs. That will potentially cause damage to the stock wiring harness and housing due to the increased wattage. Use the same wattage bulbs. If you're sticking with the bulb route, look into HIR bulbs.
For the warranty to be voided, Toyota has to prove that modification you've done caused the part(s) to fail. So installing a stereo system in your car, Toyota can't deny your warranty claim if the transmission fails.