As a cation-deficient, p-type semiconductor, copper sulfide (Cu2-xS) shows promise for applications such as photovoltaics, memristors, and plasmonics. However, these applications demand precise tuning of the crystal phase as well as the stoichiometry of Cu2-xS, an ongoing challenge in the synthesis of Cu2-xS materials for a specific application. Here, a detailed transformation diagram of cation-exchange (CE) chemistry from cadmium sulfide (CdS) into Cu2-xS nanowires (NWs) is reported. By varying the reaction time and the reactants' concentration ratio, the progression of the CE process was captured, and tunable crystal phases of the Cu2-xS were achieved. It is proposed that the evolution of Cu2-xS phases in a NW system is dependent on both kinetic and thermodynamic factors. The reported data demonstrate that CE can be used to precisely control the structure, composition, and crystal phases of NWs, and such control may be generalized to other material systems for a variety of practical applications.