can't simply switch out lower units. Might be possible to convert and use your manual powerhead on an electramatic motor, if the electramatic is otherwise intact, but probably no reason to do that if you have a good manual runner.
just separate animals. Both good systems. If the electric lower unit is available, might grab it for future reference -- use it when someone gives up on their electramatic motor. Most people know their way around a manual model, not so much the electramatic. (also need the shift box if putting a rig together).