We identify a first-order, isosymmetric transition between a ferrielectric (FiE) and ferroelectric (FE) state in A-site ordered LaScO3/BiScO3 and LaInO3/BiInO3 superlattices. Such a previously unreported ferroic transition is driven by the easy switching of cation displacements without changing the overall polarization direction or crystallographic symmetry. Epitaxial strains less than 2% are predicted to be sufficient to traverse the phase boundary, across which we capture a ≈5× increase in electric polarization. Unlike conventional Pb-based perovskite ceramics with a morphotropic phase boundary (MPB) that show polarization rotation, we predict an electromechanical response up to 102 pC/N in the vicinity of the FiE-FE phase boundary due to polarization switching without any change in symmetry. We propose this transition as an alternative ferroic transition to obtain a piezoelectric response, with the additional advantage of occurring in benign chemistries without chemical disorder.