Pertaining to the Woodcraft cigar pens, I read somewhere, either on the website, or in the new instructions, that the pens had been essentially 'improved'. You no longer had to pull the barrels apart to change the cartridges. The approved method was to remove the nib to change the cartridge. Once you press the barrels together, it was supposed to be a permanent thing.
Not an improvement to me. Some of my nibs are vary tight and difficult to loosen.
You can, however, just unscrew the upper barrel from the lower, leaving the transmission in place in the upper barrel.