Transformative and reconstructive reactions impart significant structural changes at particle boundaries of hybrid perovskites, which influence environmental stability and optoelectronic properties of these materials. Here, we investigate the moisture-induced transformative reactions in formamidinium(FA)-based perovskites FAPbX3 (X = I, Br) and show that the ambient stability of these materials can be adjusted from a few hours to several months. For FAPbI3, roles of water vapor, particle size, and light illumination on the kinetic pathways of the cubic (α) transformation to the hexagonal (δ) phase are analyzed by X-ray diffraction, optical microscopy, photoluminescence, and solid-state (ss) NMR spectroscopy techniques. The grain and subgrain boundaries exhibit different α- → δ-FAPbI3 phase transformation kinetics. Our study suggests that the dynamic transformation involves the local water-induced dissolution of the cubic phase occurring at the crystal surfaces followed by precipitation of the hexagonal phase. Insights into structures and dynamics of a kinetically trapped α-|δ-FAPbI3 are obtained by 1H, 2H, and 207Pb ssNMR spectroscopy.