I got one of these off last week but it took a bit of effort! I tried pulling it off by using pliers towards the rear of the knob. This loosened it so after that I just wiggled it for a good 15 minutes or so, gave it a final yank and it flew off and landed on the rear seat
It didn't break or crack but was slightly chewed around the rear end but this does not notice when fitted to the new heater. Not sure if this method will work with every knob though - I suppose it depends on how solidly yours is fixed as it may have been glued on at some point in the past.
Not very scientific, and there may well be an easier way, but you could give this a go. Good luck!