Optical transmittance change and stroboscopic micrographs have been obtained in homogeneously aligned thin cells of S-MHPOBC. C_8H_<17>O-〓--〓-COO-〓-COO-^*CH(CH_3)C_6H_13, and R-TFMNPOBC, C_8H_<17>O-〓--〓-COO-〓-COO-^*CH(CF_3)C_8H_<17>, by varying the initial applied DC voltage stepwise into the final one across the threshold. There exist two components. fast and slow, in the transmittance change due to the phase transition from antiferroetectric SmC^*_A to ferroelectric SmC^*. The fast component is due to the pretransitional effect. The movement of the domain boundaries is responsible for the slow component. In the transmittance change due to the transition from SmC^*_A to C^*_A, only the slow component is observed. The movement speed is mainly determined by the difference between the final and threshold voltages.
We have shown that field-induced antiferro-ferroelectric phase transition is accompanied by reversible smectic layer switching. A layer structure without any electric field applications forms a chevron structure of the same kind as in ordinary SmC^*. After the application of a field, the field-induced bookshelf structure relaxes to an obtuse chevron. structure. The texture change with focal conics or zigzag defects is interpreted by taking account of the field dependence of the layer tilt angle of the chevron structure. The origin of the layer switching is also discussed on the basis of the texture change.