Recent experiments have reported difficulties in nucleating the transformation of domains during the hcp to fcc phase transition in cobalt at 400 °C. By writing the energy associated with these domain patterns explicitly in terms of anisotropy, magnetostatic pole, and domain‐wall energies, in addition to the energies associated with an applied external field and domain‐wall pinning, a condition of instability, i.e., a free transformation of the domain structure, is obtained. It was found that the field needed to nucleate this domain transformation is increased by the effects of magnetostatic energies and wall pinning, but decreased by the effects of anisotropy. Furthermore, it was found that for an internal planar defect, such as a stacking fault, the transformation between the domain structures indigenous to each crystal phase occurs spontaneously in agreement with experimental observations.