There's no way to forfeit warranty on them everyone would do it if so and alot of phones these days the bootloader is forever locked how we have come to accept this I don't know imagine not being able to install whatever os you want on your PC it's the same thing.
And I don't 100% agree with voiding the warranty for puting their older software back on anyway.
Yeah fine void it I put custom fw that makes the blu-ray spin at 50x and blows it up but why should the warranty be void for going back to an old official version.
Many people in the vista days had xp installed at the store and they kept the hardware warranty its the same thing.