Somebody did a tutorial and by the looks of it, it's much harder than I imagined.
http://he-man.us/Innovations/MOTUC-C...KingHssss.html
This isn't a factory error as such but a molding error (Mattel). The R & L stamps are on the wrong shoulders so the factory guy was actually doing his job properly. The problem now comes that despite the shoulders being reversed, the pegs for Left and Right are different so one would need to modify the pegs in order to swap the shoulders around.
The error here is fundamental and it would need you to 'kitbash like' alter the joints of the figure in order to 'correct' it.