Perhaps they are thinking of using the Airbus Cross Crew Qualification and Mixed Fleet Flying provisions to crew the A320 by A330 Captains in the interim period until they get the recruitment/upgrade numbers sorted out for the A320.
This would also protect against having New Captains (upgrades/DEC) and New FOs (Cadets/New Recruits) being rostered on the same flights.
Alternatively having an A330 FO upgrade via a A320 command wouldn't necessarily be a bad thing provided the path to A330 Command is short. It is also good for new command upgrades to hone those command skills on regional sectors.
Ultimately it would be more preferable to have promotions within the group than the DEC option.