The recent discovery of negative Poisson's ratio (NPR) in two-dimensional (2D) atomic crystals has stimulated extensive research interest in their intriguing physical properties. Here, via density functional theory (DFT) calculations, we reveal in the family of 2D IV-VI semiconductors that an iso-symmetry structure variation concerning the switch of the cation (IV) versus anion (VI) in the outermost layers leads to the change of sign of Poisson's ratio. Such iso-symmetry structural pseudo-phase transition can be induced by external strains, as well as electric fields, realizing the possibility of an electrically switchable Poisson effect. The phase transition process could involve a dynamic intermediate state with an alternative cation/anion switch in the frequencies of 2-3 THz according to the real-time time-dependent DFT (rt-TDDFT) calculations. The results open the way for studying pseudophases in 2D materials associated with sharply different physical properties, such as Poisson's ratio, for electromechanical and optoelectronic applications.