When I Was New To All These Stuff, I Flashed My k790 With k818 Firmware And It Lost Its Signals. I Didnt Knew About GDFS So I Mistakenly Flashed A GDFS From My Friends Phone And My Phone Turned Into A Brick. Although It Was My Own Mistake But The Company Swaped My Phone With A New One.
In Short, Warranty Doesn't Voids After Flashing.