Depending on the diameter of the poles and length they usually can be changed as long as some other replacement is compatible. When exchanging threaded pole pieces, it is best to use the same diameter screw with the same TPI (threads per inch) and by using shorter or longer screws can slightly change the tonality of the pickup. Much has to do with the magnet, magnetic pattern, and the material of the pole pieces. Make sure you don’t buy a brass screw as it will not conduct the magnetic field to the strings. Some screws are plated to look nickel or chrome but are non-ferrous underneath. Pole pieces are in a pickup to direct the magnetic field to the strings which in turn vibrate and move the magnetic field through the coils and generate an alternating current that travels through the various controls and to the amplifier.