  Given a flat-foldable origami crease pattern $G=(V,E)$ (a straight-line
drawing of a planar graph on a region of the plane) with a mountain-valley (MV)
assignment $\mu:E\to\{-1,1\}$ indicating which creases in $E$ bend convexly
(mountain) or concavely (valley), we may \emph{flip} a face $F$ of $G$ to
create a new MV assignment $\mu_F$ which equals $\mu$ except for all creases
$e$ bordering $F$, where we have $\mu_F(e)=-\mu(e)$. In this paper we explore
the configuration space of face flips for a variety of crease patterns $G$ that
are tilings of the plane, proving examples where $\mu_F$ results in a MV
assignment that is either never, sometimes, or always flat-foldable for various
choices of $F$. We also consider the problem of finding, given two foldable MV
assignments $\mu_1$ and $\mu_2$ of a given crease pattern $G$, a minimal
sequence of face flips to turn $\mu_1$ into $\mu_2$. We find polynomial-time
algorithms for this in the cases where $G$ is either a square grid or the
Miura-ori, and show that this problem is NP-hard in the case where $G$ is the
triangle lattice.
