You can change the pole pieces as long as they are a ferrous material (conduct a magnetic field). Do not use brass or non-ferrous stainless steel screws unless you want less output from the strings. Try keeping the same thread and diameter of the screw so you don’t damage your bobbin. The stud side of a humbucker carries a large portion of magnetic field to the strings. Using shorter pole piece screws will slightly brighten the sound of the pickup and longer screws tend to soften the sound. Using longer pole piece screws extend the magnetic field further out the bottom of the pickup and away from the strings.