I know that retrofitting has been done before to Enviro 300-bodied models, either a Scania or MAN chassis and been fitted with the Cummins 6.7 engine instead.
Where a larger engine is being replaced with a smaller unit, I guess significant fabrication work needs to take place for this to happen in most circumstances.
I’m unsure how it goes if a smaller unit was removed to fit a larger unit, given that tail length regarding bodywork often increases for a chassis with a larger engine.
It would be much more difficult to take the 400H, remove its 4.5ltr engine, electric motor, rear axle & batteries then refit a 6.7ltr engine, angle drive, gearbox and new rear axle if the chassis hasn’t been design to accommodate it.