  Given a loop-free graph $H$, the reconfiguration problem for homomorphisms to
$H$ (also called $H$-colourings) asks: given two $H$-colourings $f$ of $g$ of a
graph $G$, is it possible to transform $f$ into $g$ by a sequence of
single-vertex colour changes such that every intermediate mapping is an
$H$-colouring? This problem is known to be polynomial-time solvable for a wide
variety of graphs $H$ (e.g. all $C_4$-free graphs) but only a handful of hard
cases are known. We prove that this problem is PSPACE-complete whenever $H$ is
a $K_{2,3}$-free quadrangulation of the $2$-sphere (equivalently, the plane)
which is not a $4$-cycle. From this result, we deduce an analogous statement
for non-bipartite $K_{2,3}$-free quadrangulations of the projective plane. This
include several interesting classes of graphs, such as odd wheels, for which
the complexity was known, and $4$-chromatic generalized Mycielski graphs, for
which it was not.
  If we instead consider graphs $G$ and $H$ with loops on every vertex (i.e.
reflexive graphs), then the reconfiguration problem is defined in a similar way
except that a vertex can only change its colour to a neighbour of its current
colour. In this setting, we use similar ideas to show that the reconfiguration
problem for $H$-colourings is PSPACE-complete whenever $H$ is a reflexive
$K_{4}$-free triangulation of the $2$-sphere which is not a reflexive triangle.
This proof applies more generally to reflexive graphs which, roughly speaking,
resemble a triangulation locally around a particular vertex. This provides the
first graphs for which $H$-Recolouring is known to be PSPACE-complete for
reflexive instances.
