Block copolymer polymersomes offer considerable access for applications in a variety of fields; however, the traditional cosolvent self-assembly method can only produce polymersomes at a low solids content (typically <1%). Recently, an in situ growth method, termed polymerization-induced self-assembly (PISA), has been developed to allow the preparation of polymersomes at high solids (10–50%). Synthesis and self-assembly of block copolymers occur simultaneously in PISA, and therefore, morphological evolution occurs throughout the polymerization. It is highly desirable to provide mechanistic insights into morphological evolution that enables one to rationally synthesize a variety of morphologies. Herein, we demonstrate that the further growth of polymersomes in aqueous PISA can be conveniently driven by temperature via two different pathways: (i) at high temperatures from polymersomes with a thin membrane to polymersomes with a thick membrane and (ii) at low temperatures from spherical polymersomes to tubular and donut-like polymersomes. We show that both the hydrodynamic diameter and membrane thickness of polymersomes increase during PISA at high temperatures, indicating that the membrane of polymersomes grows inward and outward simultaneously as the polymerization proceeds. Furthermore, careful characterization of samples withdrawn during the kinetic study at low temperatures by transmission electron microscopy and dynamic light scattering reveals various intermediate morphologies that provide important insights into the formation of tubular and donut-like polymersomes. We expect that this study not only provides mechanistic insights into the morphological evolution of PISA but also expands the scope of PISA for the preparation of a variety of structures.