You were told the incorrect information I am afraid.
Once a flight is “flown” you will automatically be classed as a no show,most airlines don’t let you change the ticket once thi sscenario has occured. Those that do categorise this as “changes after departure” which is usually a much higher fee.
I would be looking to seek some compensation from CX based on the information they have informed you over the phone as it was incorrect. The ground staff were well within the rights to charge this as per the fare rules.