I think the gov have, as so often, got themselves in a mess trying to reconcile 2 contradictory bits of legislation.
Reminds me of when Reliants had to have reverse gear blanked off in order to be able to drive them on a bike licence (coz bikes don't have reverse). Then there started to be bikes with reverse (Russian, for when a sidecar was attached) and they had to let Reliants have reverse.
In any case, why would you want to put old plates on a vehicle which didn't have that style? Not authentic in my book!