It's been an exercise in patience and systematic approach....
The black knobs went off simply by pulling. The red one did not want to move at all. I decided to heat it. I first covered the back with a 'shell' of paper tape. It's NOT sticking to the back panel, so that no heat is transferred. Then carefully heating the knob with a gun, but in multiple steps of say 30 seconds so that the heat has time to reach the metal slider inside and can loosen up. If you try to warm it up in one go the plastic will melt. It took about 20-30 cycles to get it moving a bit. Then I used a plastic tool (for trim removal), resting on a metal backplate to pop it off.
