  The ferromagnetic phase of Co$_3$Sn$_2$S$_2$ is widely considered to be a
topological Weyl semimetal, with evidence for momentum-space monopoles of Berry
curvature from transport and spectroscopic probes. As the bandstructure is
highly sensitive to the magnetic order, attention has focused on anomalies in
magnetization, susceptibility and transport measurements that are seen well
below the Curie temperature, leading to speculation that a "hidden" phase
coexists with ferromagnetism. Here we report spatially-resolved measurements by
Kerr effect microscopy that identify this phase. We find that the anomalies
coincide with a deep minimum in domain wall (DW) mobility, indicating a
crossover between two regimes of DW propagation. We demonstrate that this
crossover is a manifestation of a 2D phase transition that occurs within the
DW, in which the magnetization texture changes from continuous rotation to
unidirectional variation. We propose that the existence of this 2D transition
deep within the ferromagnetic state of the bulk is a consequence of a giant
quality factor for magnetocrystalline anisotropy unique to this compound. This
work broadens the horizon of the conventional binary classification of DWs into
Bloch and N\'eel walls, and suggests new strategies for manipulation of domain
walls and their role in electron and spin transport.
