The pics not quite clear to me. If it has a circlip, then two small screwdivers can be used to prise the retaining clip out of the slot and the cog slips off. A threaded driver/cog is fussier. You can dismantle the assembly, clamp the driver in a bench vise, then use heat and chain whip.
OP, maybe a 50's hub without the threaded driver is a better option if you want to swap cogs?