I did one. Don't remember exactly how it went, but wasn't particularly difficult, just HEAVY and awkward.
So the pulley and drum spins on the stationary shaft, and you have the whole mess off of the mower frame, but no way to get the shaft separated from the drum. I THINK I blocked it up horizontally, put the nut back on flush with the top of the threads, propped an oak 2x8 against the nut and smacked it with a sledge hammer. Plywood would work fine too.