It depends, for pads on an old rotor surface it is generally not advised to simply replace the brake pads. New pads on worn rotors can create issues with the new pads, cause vibration, or make braking less safe than with new pads and new rotors together. This means mechanics may need to address the rotors at the same time. Brake rotors and calipers are more expensive and often more complicated to replace than just pads.