  We present a method for generating suites of dark-matter halo catalogs with
only a few $N$-body simulations, focusing on making small changes to the
underlying cosmology of a simulation with high precision. In the context of
blind challenges, this allows us to reuse a simulation by giving it a new
cosmology after the original cosmology is revealed. Starting with full $N$-body
realizations of an original cosmology and a target cosmology, we fit a transfer
function that displaces halos in the original so that the galaxy/HOD power
spectrum matches that of the target cosmology. This measured transfer function
can then be applied to a new realization of the original cosmology to create a
new realization of the target cosmology. For a 1% change in $\sigma_8$, we
achieve 0.1% accuracy to $k = 1h\,\mathrm{Mpc}^{-1}$ in the real-space power
spectrum; this degrades to 0.3% when the transfer function is applied to a new
realization. We achieve similar accuracy in the redshift-space monopole and
quadrupole. In all cases, the result is better than the sample variance of our
$1.1h^{-1}\,\mathrm{Gpc}$ simulation boxes.
