Hi.
I've said it before and I'll say it again:
If a plastic splined shaft knob won't budge with moderate force, break it. Unless it's a vintage one, in that case break the potentiometer instead.
Splined shaft potentiometers aren't IME designed for changing the knobs. Too much axial force, and the wiper plate will detach itself from the shaft.
IF OP has a set-screw knob on a solid shaft, the job may become easier.
Just my 0.02
Sam
You suggest breaking something in order to repair it.
You also advise breaking a vintage pot to spare a vintage knob.
Is this the correct reading of your advice?
about if this is a serious question 