  Much excitement surrounds the intense mass loss that seems to take place in
some massive stars immediately before core collapse. However, occurring too
late, it has a negligible impact on the star's evolution or the final yields,
which are influenced instead by the longer-term, quasi-steady mass loss taking
place during H and He burning. Late-time observations of core-collapse
supernovae interacting with the progenitor wind are one means to constrain this
secular mass loss. Here, we present radiative transfer calculations for a Type
II SN with and without this interaction power, focusing on the phase between
350 and 1000d after explosion. Without interaction power, the ejecta are
powered through radioactive decay whose exponential decline produces an
ever-fading SN. Instead, with a constant interaction power of $10^{40}$ erg
s$^{-1}$ (representative of an SN II ramming into a steady-state $10^{-6}
M_\odot$ yr$^{-1}$ wind), the spectrum morphs from decay powered at 350d, with
narrow lines forming in the inner metal-rich ejecta, to interaction powered at
1000d, with broad boxy lines forming in the outer H-rich ejecta. Intermediate
times are characterized by a hybrid and complex spectrum made of overlapping
narrow and broad lines. While interaction boosts primarily the flux in the
ultraviolet, which remains largely unobserved today, a knee in the $R$-band
light curve or a $U$-band boost are clear signatures of interaction at late
times. The model predictions compare favorably with a number of Type II
supernovae including SN 2004et or SN 2017eaw at 500-1000d after explosion.
