There's "True North" and "Magnetic North". If the poles reversed, compasses would still work, just in reverse, North would be South. Magnetic North, depending on where you live in the world, is off by so many degrees. If memory serves since I was in flight school, anywhere from 2 to 7 degrees. Here where I live it's off positive 4 degrees. If polarity reverses, True North and Magnetic North would be opposite, which would take quite a bit more calculations along with the confusion.....