About the only thing I’ve recently read, on the brakedirect web site Cuda posted, is that a change of pad compound should be accompanied with machining of the rotors to ensure the new pad compound beds into a clean rotor face so it works as intended and without old compound contamination causing problems.
But in the past, if the rotor was not scored or didn’t have a lip on it, I’ve simply sanded the rotor face with emery cloth and then used brake cleaner to get them spotless. Is this a wrong approach?