I thought the date for vintage was set by the Society of Ploughmen and applies to all classes.
The introduction of more modern small tractors, if it happens, should be a new class leaving the current Vintage Horticultural class as it is, ie primarily 2 wheeled walk behind tractors etc. The list of eligible vintage tractors does not need amending. Perhaps the small tractors should compete in one of the existing tractor classes as do grey Ferguson tractors.