just bring it in, look for the dumbest looking worker, tell them it started scratching and not reading disks and you want a new one. Did it with my ps3 at gamestop.
they cant deny you a store warrenty. i went thru 3 xbox 360s doing this aswell. started with a 20gig and now i have an elite, without spending any extra money. i sold them my warrenty back like 3weeks befor it was up lol. oh, and if your slick enuff you can gank a few things like the head set and hdmi cable (if you have it). they only look at the 360 and hard drive