Materials showing field-driven antiferroelectric to ferroelectric transformation are interesting for the design of pyroelectric, high strain actuator and high-power dielectric energy storage and discharge devices. The stability of the antiferroelectric and ferroelectric states at room temperature can be chemically tuned by La and Ti co-modification of the antiferroelectric system Pb(Zr1-ySny)O3. Here, we have chosen a composition Pb0.982La0.012(Zr0.60Sn0.30Ti0.10)O3 (PLZST) at the threshold of the antiferroelectric-ferroelectric instability at room temperature and investigated the effect of electric field, temperature and pressure on the stability of the structural-polar states. We show that electric poling transforms irreversibly the antiferroelectric state into a ferroelectric state. Application of hydrostatic pressure ~100MPa on the field-stabilized ferroelectric phase, however, restores the antiferroelectric state and depolarizes the system.