The electric field-induced antiferroelectric-to-ferroelectric phase transition is investigated through detailed measurements of electric polarization P, longitudinal strain x(33), and transverse strain x(11) developed under applied electric fields in a series of Pb0.99Nb0.02[(Zr0.57Sn0.43)(1-y)Ti-y](0.98)O-3 ceramics with compositions close to the antiferroelectric/ferroelectric phase boundary. It is found that the volume expansion, expressed as (x(33)+2x(11)), at the antiferroelectric-to-ferroelectric phase transition remains similar to 0.4% regardless of the composition in the range of 0.060 < y < 0.075. However, the induced ferroelectric phase in compositions y >= 0.069 becomes metastable and the ferroelectric-to-antiferroelectric phase transition does not occur during the unloading of the applied field. This reverse phase transition occurs partially when electric fields with reversed polarity are applied. As a consequence, the switchable mechanical strains in compositions y >= 0.069 are significantly reduced as y (Ti content) increases even though the switchable polarization remains at a high value.