Purely from a numerical point of view in terms of the number of vehicles the following would be possible, which would give a fleet of 21 x 5-car 333's:
All 16 non-driving vehicles are removed from 4-car 332001-004 and 010-013 and refurbished, with the current fleet of 333001-016 each receiving 1.
5-car 332005-009 remain in their current formations, are refurbished and become 333017-021.
All driving cars from 332001-004 and 010-013, as well as all 4 vehicles from the long-demic 332014 are sent to the rolling stock graveyard.
Obviously won't happen due to a million and one other reasons, but would have worked numerically.
Cheers.