Later Stringmasters have one of the two pickups in each neck reverse wound and reverse polarity. This will make the pickups humbucking and quiet when both pickups are used together. The blend switch located behind the bridge saddle on each neck and can blend in the 2nd pickup when fully turned clockwise and only one pickup when fully turned counter-clockwise. This is a great setup and I often use this on conventional electric guitars as well.

On your current setup the magnets would have to be reverse magnetized and the coil would have to be reverse wound or the wires reversed. If you reverse the wires when using a metal cover I would insulate the magnets so they don’t accidentally short out to the grounded cover. I would measure the existing gauss or strength of the magnetic field before the pickup is magnetized. If it is currently south polarity on top of the pickup I would remagnetize it north polarity. I would then recalibrate it to it’s previous gauss.