To change it on an A-320 depends on versions installed. Some of the newer ones let you get back into Init page A inflight so the ID can be modified.
If this function is not available you could try using the secondary plan and delete current, then redo the whole lot including INIT pages and then activate secondary. I have not tried it but have heard it done but do not know the result. There is a note of caution doing it this way as the result may not go the way you want.
The ID should be exactly as filed on the flight plan as this is what ATC insert into their system.