Abstract: A speed changer assembly for changing the rotational speed of an engine's (or other prime mover's) output to that required by the input of a process machine. In operation, a first gear is in meshing engagement with an idler gear, which in turn, is in meshing engagement with a second gear. When operation of the speed changer assembly is halted, the idler gear is pivotable about one of the first and second gears. A center distance between the idler gear and the gear about which the idler gear is pivoted remains constant while the center distance between the idler gear and the other gear is varied. The idler gear is taken out of meshing engagement with the other gear, permitting replacement of said other gear. The idler gear is then pivoted to establish meshing engagement with the replacement gear and the speed changer is configured with a new speed ratio.