My initial thought is that it would be weird having a racing season that doesn't align with the breeding season and the horses birthday (assuming that Adam is not also proposing a change to that). With the major races over in December what would our juvenile horses do for the rest of the season?
December is way too early for 2YO races under the present breeding season arrangements.
If he is also advocating that the breeding season be changed to the calendar year, a gamebreaker would be the barrier that would create for dual hemisphere stallions.