With vintage style Fender tele bridge pickups, or even Fender tele neck pickups, or strat pickups, or any passive single coil vintage style pickup that uses rod magnets inset into the bobbin, flipping the top magnetic polarity is super easy, fast, and cheap to accomplish. It can be done with the pickup already installed on the guitar, just move the strings out of the way to allow the magnet to touch the rod magnet pole piece.

You'll need a cheap magnetic compass and a neodyminum (rare earth) magnet, as shown in the images below. Big box stores like Home Depot sell a six pack of 1/2" rare earth magnets for about $9 - use the entire stack of magnets for charging pickup rod magnets.


First, find out the top polarity of the pickup you wish to flip. the south compass needle points to the top of the pickup, so this Tele bridge bobbin is top magnetic NORTH ...



Now to find the north side of the neo mag ...



Touch the north side of the neo mag to the top of each of the six bobbin pole pieces for a few seconds each, from one side ...



to the other ...



And now that north top magnetic bobbin is now top magnetic SOUTH ...