Brian, an ECU is a complete engine assembly with all the extra components fitted to the basic engine. Think a car engine and then the water pump, alternator, head box etc. A 'Simple' engine change involves removing the engine, stripping all the ancilliaries off it, refitting them to the new engine and reinstalling it. Changing an ECU, all out all in. The paperwork is probably simpler too as all the components will have been serviced and service intervals reset.
We once needed an engine change overseas. The engine was removed and packaged by an aptly named Sgt Hammer. The replacement was delivered, ours was loaded on the transport and back to UK. When we unpacked the engine we found just an engine and not a complete ECU. So we had another week in the Sun.