To be fair, a game of switcheroo doesn't have to involve a complete shift. It doesn't have to evolve into a full-scale alien invasion, when a suitable scenario might just be some bandits who found a couple nearly empty laser pistols on some desiccated alien corpses in some wreckage out in the desert. It doesn't have to be a zombie apocalypse, if it's just a crooked undertaker raising a lone zombie to scare folks and steal their stuff. And so forth. A western campaign where the supernatural is intermittent and mixed in with the mundane wouldn't have to require too many outre rulings beyond the scope of the regular rules. Seems to me like the main bone of contention might just be how focused the system is on pure gunfighting versus everything else.