It depends on the terms of your particular warranty, but I wouldn't suspect a warranty to be voided normally. Barring any technical issues, you can always flash back the previous BIOS. Just make sure you back up the current, working BIOS, before making changes, and it might help to understand the procedure to repair the card, in the case of a bad BIOS flash.
negative, the ln2 is made for flashing. as long as the original bios is detected on the classified, evga will honor the warranty. hell, you can even remove the stock cooler and throw on an ekwb block, use the classified voltage tool app to push the core to 1.35v, and run 1400-1500mhz on the core, kill the card, and again, as long as the bios switch 1 has the original bios, they will honor the warranty.
people on the classified forums over at overclock.net have well confirmed this.
this is why evga is king and no other board partner comes close to warranty or customer service.