Nonequilibrium processes in thin films of lamella-forming poly(2-vinylpyridine)-block-polyisoprene diblock copolymers were studied. The thin films having the half-lamella thickness were prepared by spin-casting of a block copolymer solution onto a hydrogen-terminated silicon wafer (SiH). We found that (1) the as-spun films develop a perpendicularly oriented microdomain structure, i.e., microphase-separated structure with its microdomain interfaces perpendicular to the film surface, and (2) after thermal or solvent vapor treatment, this trapped nonequilibrium structure is transformed into parallel lamellae (lamellar microdomains with their lamella interfaces parallel to the surface) by an orientational transformation process without changing the mean film thickness.