Here's a diagram that shows the mod - this was posted at the Les Paul Forum some time ago and I kept a link to it. As I understand it the mod puts the tone pot after the volume pot, the effect being to retain more highs (than modern wiring) as the volume is rolled down.
I believe the second part (swapping the wires on the tone pot) is optional, i.e. cosmetic only to be historically "correct". The first part actually makes the difference.