The pathway and kinetics for the cylinder-to-sphere order-order transition in a mixture of a matched diblock and triblock copolymer of styrene and ethylene-butene-1 is reported. The microstructure transformation was monitored by viscoelastic measurements, and the structural assignments of the intermediate states were performed by electron microscopy. The kinetics of transformation from macroscopically unaligned wormlike cylindrical microdomains to spherical microdomains arranged on a bcc lattice were extremely slow. The wormlike cylinder-to-sphere transition slowed with decreasing quench depth from the order-disorder transition. Additionally, these order-order kinetics were in quantitative agreement with those for the development of spherical microdomains from an initial disordered state. Further, during the initial induction time following a temperature jump from a wormlike cylindrical order to a spherical state, the sample exhibited liquidlike viscoelastic characteristics and structurally showed the absence of long-range order. In contrast, shear-aligned cylinders rapidly transformed to spheres, adopting a viscoelastic pathway distinct from that of the unaligned samples. The cylinder-to-sphere transition is thermotropically reversible, with the viscoelasticity-based kinetics of the sphere-to-cylinder transition being slower than the cylinder-to-sphere transition.