For big Boeings and Airbuses, thought that moving a stored flight plan to the active flight plan was done via the MCDU.
Didn't know that operating the transponder could cause a stored flight plan to become active (see below). Can the transponder code changing described below also be done via the MCDU?
"Certainly not outside the realms of possibility that, whilst transitioning from one code to another, that another flight-plan could be moved from "announced" to "active"".
Aviation Today :: Changing Transponder Codes