  We examine the effect of neutrino flavor transformation by the fast flavor
instability (FFI) on long-term mass ejection from accretion disks formed after
neutron star mergers. Neutrino emission and absorption in the disk set the
composition of the disk ejecta, which subsequently undergoes $r$-process
nucleosynthesis upon expansion and cooling. Here we perform 28 time-dependent,
axisymmetric, viscous-hydrodynamic simulations of accretion disks around
hypermassive neutron stars (HMNSs) of variable lifetime, using a 3-species
neutrino leakage scheme for emission and an annular-lightbulb scheme for
absorption. We include neutrino flavor transformation due the FFI in a
parametric way, by modifying the absorbed neutrino fluxes and temperatures,
allowing for flavor mixing at various levels of flavor equilibration, and also
in a way that aims to respect the lepton-number preserving symmetry of the
neutrino self-interaction Hamiltonian. We find that for a promptly-formed black
hole (BH), the FFI lowers the average electron fraction of the disk outflow due
to a decrease in neutrino absorption, driven primarily by a drop in electron
neutrino/antineutrino flux upon flavor mixing. For a long-lived HMNS, the disk
emits more heavy lepton neutrinos and reabsorbs more electron neutrinos than
for a BH, with a smaller drop in flux compensated by a higher neutrino
temperature upon flavor mixing. The resulting outflow has a broader electron
fraction distribution, a more proton-rich peak, and undergoes stronger
radiative driving. Disks with intermediate HMNS lifetimes show results that
fall in between these two limits. In most cases, the impact of the FFI on the
outflow is moderate, with changes in mass ejection, average velocity, and
average electron fraction of order $\sim 10\%$, and changes in the
lanthanide/actinide mass fraction of up to a factor $\sim 2$.
