Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr_0.2Ti_0.8O₃ thin films is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that, under an electric field induced between the PFM tip and the bottom electrode, the 180° switching occurs in both c and a domains. After the removal of this field, the spontaneous reversal of the out-of-plane and in-plane polarizations back to the initial orientations takes place, evolving via heterogeneous development of antiparallel 180° domains. The switching of in-plane polarization inside a domains and the preferential formation of reversed 180° domains at 90° domain walls are explained by the effects of the depolarizing fields caused by transient polarization charges appearing on these domain walls.