  Black-hole X-ray binaries exhibit different spectral and timing properties in
different accretion states. The X-ray outburst of a recently discovered and
extraordinarily bright source, Swift$~$J1727.8$-$1613, has enabled the first
investigation of how the X-ray polarization properties of a source evolve with
spectral state. The 2$-$8 keV polarization degree was previously measured by
the Imaging X-ray Polarimetry Explorer (IXPE) to be $\approx$ 4% in the hard
and hard intermediate states. Here we present new IXPE results taken in the
soft state, with the X-ray flux dominated by the thermal accretion-disk
emission. We find that the polarization degree has dropped dramatically to
$\lesssim$ 1%. This result indicates that the measured X-ray polarization is
largely sensitive to the accretion state and the polarization fraction is
significantly higher in the hard state when the X-ray emission is dominated by
up-scattered radiation in the X-ray corona. The combined polarization
measurements in the soft and hard states disfavor a very high or low
inclination of the system.
