  We present a detailed timing study of the pulse profile of Swift J0243.6+6124
with HXMT and Fermi/GBM data during its 2017 giant outburst. The double-peak
profile at luminosity above $5\times10^{38}$erg\,s$^{-1}$ is found to be 0.25
phase offset from that below $1.5\times10^{38}$erg\,s$^{-1}$, which strongly
supports for a transition from a pencil beam to a fan beam, and thus for the
formation of shock dominated accretion column. During the rising stage of the
high double-peak regime, the faint peak got saturated in 10-100 keV band above
a luminosity of $L_t\sim1.3\times10^{39}$erg\,s$^{-1}$, which is coincident
with sudden spectral changes of both the main and faint peaks. They imply a
sudden change of emission pattern around $L_t$. The spin-up rate ($\dot{\nu}$)
is linearly correlated with luminosity ($L$) below $L_t$, consistent with the
prediction of a radiation pressure dominated (RPD) disk. The $\dot{\nu}-L$
relation flattens above $L_t$, indicating a less efficient transfer of angular
momentum and a change of accretion disk geometry above $L_t$. It is likely due
to irradiation of the disk by the central accretion column and indicates
significant radiation feedback before the inner disk radius reaching the
spherization radius.
