The knob is attached to the brass shaft with epoxy.
To remove it, you will need to heat it to around 240F to soften the epoxy.
Use a Bic lighter and heat for about 6 - 10 seconds. Not too hot, just enough to get the epoxy to loosen up, or the control may be damaged.
Best to heat the knob and try to turn it (with gloves or a rag when hot) than to pull on it. Pulling hard on the shaft can cause problems, so try and get it to spin free first.
Once you get the knob off, and remove the heatsink, you will also need to remove the two screws inside the battery tube holding the switch assembly in place. Then push the switch assy out the rear of the battery tube.
The setscrew for the heat sink is 6-32 thread, so you'll need to drill/tap a hole in the new battery tube before assembly.
If you have any more questions, let me know. - I'm kind of slow checking email and this site nowadays. but do check back at least every couple of weeks, so if I don't answer for a while, I'm off visiting friends and family somewhere, but will answer eventually.