The easiest way that I found was to carefully wrap masking tape around the edge of the knob to protect it from scratching. I carefully held it in a small hand vice and took my soldering iron and carefully held it to the metal shaft. As the shaft would heat up and softened the plastic in the knob, I carefully pulled the shaft out using a small pair of pliers. It seems that the friction of putting the knobs on causes the plastic to harden and crystallize around the knurled shaft. The older knobs were made of Butyrate which is a softer material and didn’t cause such a problem as the newer plastic materials being used.