As everyone has been saying, you're protected as long as your modification cannot be linked to a warranty claim. This protection was granted in the Magnuson-Moss Warranty Act enacted in 1975. Loosely it states:
"The federal minimum standards for full warranties are waived if the warrantor can show that the problem associated with a warranted consumer product was caused by damage while in the possession of the consumer, or by unreasonable use, including a failure to provide reasonable and necessary maintenance."
Magnuson?Moss Warranty Act - Wikipedia, the free encyclopedia