Tony, gently warming the collar will soften it enough to allow the nib to be pulled out.
Some caution is called for, though - the collars will warp if too much heat is applied. I had a unit that must have been dropped, and decided to remove the nib to reshape the tines. I used a heat gun, dialed down to "uncomfortably warm", and, well, had to scavenge a collar from something else.
Luckily, I had a nib unit from a no-name pen with the same threading, made from some clear plastic that handled the heat much better.
Still, unless the nib's rare, precious, or you don't care about the collar, I'd recommend against this.
Best,
Jon