  Comets, relics from the early solar system, consist of dust and ice. The ice
sublimates as comets approach the Sun, ejecting dust from their nuclei seen as
activity. Different volatiles sublimate at different Sun-comet distances and
eject dust of unique sizes, structures, and compositions. In this study, we
present new polarimetric observations of Oort-cloud comet C/2017 K2 (PANSTARRS)
in R and I-filter domains before, during, and after its crossover of the
water-ice sublimation regime at phase angles of 15.9\arcdeg, 10.5\arcdeg, and
20.0\arcdeg, respectively. Combining multiband optical imaging data covering a
wide range of heliocentric distances ($\sim$14$-$2.3 au), we aim to
characterize the preperihelion evolution of cometary activity as well as the
properties of its coma dust. Two discontinuous brightening events were
observed: at $\sim$6 au presumably associated with changes in CO-like
supervolatile ice activity, and at $\sim$2.9 au when water ice took over.
Particularly, the latter activation is accompanied by changes in coma
morphology and color whose trends differ between the inner ($\sim$10$^3$-km)
and outer ($\sim$10$^4$-km) parts of the coma. No polarimetric discontinuities
on the comet were observed over the inner coma region, all epochs showing
phase-angle and wavelength dependencies compatible with those of active comets
observed in similar observing geometry. During this period, the underlying dust
continuum overwhelmed H$\alpha$ emission at around 656.3 nm, suggesting less
water ice on the comet's surface than expected. We discuss K2's coma
environment by combining numerical simulations of light scattered by dust and
place the observations within the context of the comet's evolution.
