The paper considers homogeneous, cold standby systems performing missions of the fixed duration when a failure of an operating element results in a mission failure. A system is operating in a random environment modeled by the Poisson process of shocks. Each shock decreases the remaining lifetime of an operating element and, therefore, its preventive replacement using available standby element is scheduled on experiencing the predetermined number of shocks. The crucial feature of the proposed model is that the preventively replaced elements can be used afterwards as the future standby elements. The replacement is not perfect, and its probability of success decreases with the number of replacements. The number of shocks triggering elements’ replacements that maximizes the mission success probability is obtained. A numerical example with detailed analysis is presented.