I'd save the old set for sure, as they're probably perfectly suitable for running for years. There is no need to toss rotors just because you wore out some pads, in my opinion.
I'm running the original rotors on a pristine Porsche 944 right now, and they have 165,000 miles on them, they're not scored, and they brake well enough to go to the track. The Volvo XC90 is generating a lot of heat when the brakes haul the vehicle down, but even the rears are vented, and they're huge. Personally, I doubt if I would have even considered changing out the rotor.
Even in the past, when I have had scored rotors, the cheap fix was to insert a spare pad on that rotor, while changing out all the rest, and let the spare pad polish the score marks smooth (not erase them, just smooth things out) and then pop in the good pads. Unless a rotor is causing the vehicle to pull to one side or another, I don't think even scoring makes a difference, as the surface area on the rotor is still being gripped by the pad in a uniform manner.
The prev owner of my 928 scored up the rear rotors pretty badly. The car would haul down nicely, very powerful brakes, but they looked bad. I changed them out and didn't notice any difference in the way the car brakes, but those drilled rotors look cool
Ask a dealer if the rotors have to be turned every time you change pads, and they'll have to give you the liability management answer, but in reality, again in my opinion, lightly scored rotors can last many years and work fine IF you can still take your hands off the wheel and hit the brakes, and get a straight stop. Naturally, when the wear factor has them thinning out below spec, they should be replaced.