Here is how...
I have done 8 of these. It is not too difficult; just use some care. It helps to have the right tools. I finally bought a NorthWest Short Lines SensiPress. It is the perfect tool for this task. You can also improvise.
You need to make a jig on which to press the brass worm off the supplied shaft. If you have a drill press and drill press vise, that would work nicely. Invert a drill bit of smaller diameter (than the shaft) in the chuck, mount the gear vertically in the vise and use the chuck as a press arbor. The shaft will smoothly exit the gear. Do not try to pound out the shaft, it will enlarge the hole in the worm rendering it useless. If you don't have access to these tools, maybe you can improvise something similar.
The original Life-Like plastic worms are easy to remove. Just grab tightly with your fingers and pull with a slight twisting motion.
The brass worm can then be pressed onto the motor shaft. Again, you may to improvise some kind of fixture to hold the worm square to the shaft while you squeeze the shaft into the worm, maybe using a vise or drill press.
Good luck with your project. You will be pleasantly surprised at how much better your switcher will run with the brass worms. Mine now run very smoothly and quietly.
Mike
Just make sure to put jewelers rouge on the gears,before you put back on the frame.You can use a small polishing wheel on a Dremel.Then put back together again.Mine run about 2-3 ties a minute.