The bottom part of the lamp simply unscrews. The heavy foot comes off from the center pipe, the center pipe unscrews from the swivel joint. It is here where one new opening should be created. As you can see, there is plenty of room to insert a protective piece of wood or plastic while working the metal. The mains wiring must be protected from getting damaged.
If for whatever reason the mains wiring should get damaged, the lamp is not safe to operate anymore. The wiring must be repaired or replaced by a qualified person.
The next step is to remove the spring. It is a very strong one, a real bastard. Use a piece of strong string as shown in the images to pop it off. Place the string on top of the spring, close to where the hook grabs the little horizontal cross-brace. Take a firm hold of the string and lamp and pull. If it doesn't move, pull harder. Eventually it will move and you can pull it free.
Then unscrew the two tensioning nuts. This will free the rod that needs to be modified. Don't loose the thin metal washers! The remaining part of the lamp's upper body (2nd rod + swivel joint) holds the mains wiring.
Once you're done with the metal work (see later steps), re-assemble the lamp in reverse order. Make sure the mains wires are not pinched. Getting the spring back on will again require a piece of strong string. Use it to pull the spring's hook back over the cross-brace.