Save the old ones for use on your or someone else's car in a pinch (like when you are rotating tires on a Sun eve and discover you are a hair's breath form metal-to-metal contact on the brakes). New rotors always deserve new pads as your current ones will not contact them uniformly.

I recently changed the metallic Raybestos pads (Pep Boys)I installed along with new rotors on my Saab 900 18,000 miles ago. They were nearly worn out and I was experiencing significant pulsing through the pedal while braking. It was the second set of Raybestos "lifetime" warranty pads to last under 20K miles. I used PBR Deluxe this time and the pulsing nearly disappeared. I don't think the new rotors warped but I noticed there are a couple of surface imperfections on one that the other pads apparently "grabbed" on more than these do.