you have to pull it off and get lucky. I've never been lucky with pulling one off except 1 time when they are that stuck.
the last time i had to pull a knob off was in april i needed to replace the fan switch like you. i tried to pull the thing off 10 different ways to not damage the switch or the knob. nothing worked and i knew i was going to damage the face plate on the climate control system. i had all kinds of plastic pry bars pulling on that knob it would not move, put i popped off the 2 others on the defrost and heat cool controls no problem.
don't twist left and right what ever you do.
i eventually had to take a cutter plier and cut the knob off one chuck at a time until i got it off the switch so i could pull it out.
i put in a new switch and i kind of got lucky and had extra knobs somebody gave me years ago, so i pressed a new knob onto the new switch.
you can find similar knobs in the help section of most auto stores or you can find NOS knobs for sale on ebay, they might even repop them now.
you can also PM OMS (ohio mustang supply) and see if Don has some for sale.
another option is kind of more dangerous you can use a soldering iron to heat the metal the knob is on and then pull the plastic knob off by melting it slightly to save it. I've done that once and it worked but can be really dangerous(was on a parts car).
btw pulling the switch with the center dash still in place is almost impossible. you usually need to take the center dash out, i pulled my drivers seat out.
had to take the radio out and take out all the support screws, pull the dash forward from the bottom as much as possible and i made a special extension driver so i could get the one bolt out that holds the switch, then getting a new switch back in was just as awful, took me 2 days but i refused to pull the entire panel out having to pull the vaccum hose octopus out to release the entire dash.. it is 100000000% easier pulling the center dash panel to get to the switch. oh i had to pull my full floor console as well.
