This study is concerned with the morphological transition kinetics of polyelectrolyte complex micelles formed from an anionic neutral block copolymer and a cationic neutral block copolymer in aqueous NaCl solution. The transformation was induced by changing the mixing ratio of the anionic and cationic monomer units in the copolymers. The kinetics of the morphological transition was directly tracked by time-resolved (ultra) small-angle X-ray scattering coupled with rapid mixing of the block copolymer components by a stopped flow apparatus. The transformation from cylindrical to spherical micelles upon changing the mixing ratio of the copolymers was reversible, and the process occurred via the random scission of the cylindrical micelles along their contours. The reverse transition from spherical to cylindrical micelles was found to be a slow process with high activation energy.