It is more difficult, and riskier, to change from a 90 day breeding season to a 63 day breeding season than it is to change to a 42 day breeding season.
Pictured above is a young Mashona / Romosinuano cow, in good body condition, calving on green grass (Photo: Courtesy of Pepe Balderas, South Texas). Pictured below are newly born calves on green grass (Photo: Courtesy of Panco Carrillo, Chihuahua, Mexico).
Ranchers changing from 90 to 63 day breeding rely on feeding to make the change.
Ranchers changing to 42 day breeding rely on good body condition by calving on green grass to make the change.
The latter practice is much more sensible and successful. The only prerequisite for such a practice is making a mind change equivalent to breaking the sound barrier.