  We investigate the early-time dynamics of a quasi-two-dimensional spin-1
antiferromagnetic Bose-Einstein condensate after a sudden quench from the
easy-plane to the easy-axis polar phase. The post-quench dynamics shows a
crossover behavior as the quench strength $\tilde{q}$ is increased, where
$\tilde{q}$ is defined as the ratio of the initial excitation energy per
particle to the characteristic spin interaction energy. For a weak quench of
$\tilde{q}<1$, long-wavelength spin excitations are dominantly generated,
leading to the formation of irregular spin domains. With increasing
$\tilde{q}$, the length scale of the initial spin excitations decreases, and we
demonstrate that the long-wavelength instability is strongly suppressed for
high $\tilde{q}>2$. The observed crossover behavior is found to be consistent
with the Bogoliubov description of the dynamic instability of the initial
spinor condensate.
