The polarity will need to be determined and re-magnetized opposite on pickups with rod magnets. Bar magnets used in humbucking style pickups can normally be removed from a pickup and flipped over 180° and reinserted to reverse the phasing. Reversing the phasing in a pickup will only be noticed when being used with another pickup. Reversing the phase of humbuckers with 2, 3 or 4 conductor lead wire can easily be done by reversing the hot and ground wires. Sometimes there can be an internal phase problem when working with many different color combinations and hookup wires from different manufacturers.