For best results, use new pads and rotors. But the rotors could be cleaned with isopropyl alcohol.
Last Saturday a customer brought in his new ebike (bought online) because he had sprayed wd40 on his brakes to quiet them from squealing. Then he had to stop with his feet.
Anyway I replaced the pads and cleaned the rotors (he chose the cheapest option) and he returned later to say the brakes worked better than new and were quiet as well.
I wouldn’t sand the rotors but you should do the bed-in process after replacing pads.