A lowlight front panel was one piece from top to bottom, to put a late front on would mean that would have to fabricate a new in fill for the bottom to allow for the fitting of an early bumper, so it would have easier just to make some more early front panels then go about fitting late ones and making a new panel for the bottom, plus the extra fitting.
Crossovers occur in all the VW models, I don't believe they are there just to use parts up, that's what Fleetlines where for, crossovers where the half way point of major changes. The only real difference between a '72 and a '73 is the bumpers and front indicators.
1972 model year production was the most prolific year of production, nearly 300,000 Type2's where produced, I'd like to think that VW see far enough ahead not run out of panels. Plus theres the spare parts stock they'd what to have.