We present linear (AB)(n) multiblock copolymers that exhibit a thermally induced reversible alteration of the surface composition at a sharply defined transition temperature T-s of 120-170 degrees C depending on the polymer structure. At temperatures below T. the surface consists of block A, a 4,4'-methylenediphenyl diisocyanate (4,4'-MDI) type polyurea, whereas above T., the hydrophobic block B, a poly(ricinoleic acid hexanediol ester) dominates the surface composition. The ratio of surface concentrations c(A)/c(B) changes by a factor of at least 1000 within an analyzed depth of approximately 10 A. The full A-B surface transition is obtained within minutes. A mechanism is proposed where microphase crystallization of block A in the bulk effectively locks surface segregation of the hydrophobic block B, yielding an A-rich surface. The topology of the copolymers imposes sufficient restrictions for the lateral separation of the connected constituents such that surface segregation is largely reduced. Only above the transition temperature T-s of microphase crystallization of block A can block B segregate to the surface, yielding a B-rich surface. Such a scheme of competing self-organizing processes in copolymers may potentially be used to reversibly switch surface properties such as adhesion and wetting in various applications.