  We report on a detailed spectral analysis of the transient X-ray pulsar
1A~0535+262, which underwent the brightest giant outburst ever recorded for
this source from November to December 2020 with a peak luminosity of $1.2$
$\times10^{38}\ \rm erg\ s^{-1}$. Thanks to the unprecedented energy coverage
and high cadence observations provided by Insight-HXMT, we were able to find
for the first time evidence for a transition of the accretion regime. At high
luminosity, above the critical luminosity $6.7\times10^{37}$ erg s$^{-1}$, the
cyclotron absorption line energy anti-correlates with luminosity. Below the
critical luminosity, a positive correlation is observed. The 1A~0535+262
becomes, therefore, the second source after V~0332+53, which clearly shows an
anti-correlation above and transition between correlation and anti-correlation
around the critical luminosity. The evolution of both the observed CRSF line
energy and broadband X-ray continuum spectrum throughout the outburst exhibits
significant differences during the rising and fading phases: that is, for a
similar luminosity the spectral parameters take different values which results
in hysteresis patterns for several spectral parameters including the cyclotron
line energy. We argue that, similarly to V~0332+53, these changes might be
related to different geometry of the emission region in rising and declining
parts of the outburst, probably due to changes in the accretion disk structure
and its interaction with the magnetosphere of the neutron star.
