Whether you have new pads or used pads the issue is the mating surface with the rotor. Until the pads are properly bedded to the new rotors you will have only marginal braking due to the mismatched surfaces. The easiest (and fastest) way to get the used pads to bed to the new rotors is to deglaze them prior to the bedding process. This will eliminate any uneven areas, grooves or ridges as well.
Simple enough to do, carefully sand the pads surface until there is a uniform dull finish to it. Stick some 80 grit paper on a hard flat surface (concrete floor) and rub the pad on it to deglaze it being careful to keep the pad in constant even contact with the sand paper. It doesn't take much sanding to deglaze them so don't over do it. Wash them with spray brake cleaner and let them air dry before installation.
You probably won't need as many stops to bed them as you would with new pads since the resins and impurities have long since leached out of the pads.
Make sure you clean the new rotors well since many manufacturers coat them with cosmoline to prevent rust during storage.
Hope this helps.