A cold-standby redundant system with two identical units and one repair facility is considered. Units can be in three states: good (I), degraded (II), and failed (III). We suppose that only the following state-transitions of a unit are possible: I→II, II→III, II→I, III→I. The repair of a unit of the type II→I can be interpreted as a state-dependent preventive maintenance. We answer the question whether a stochastical increase of times of work or a stochastical decrease of times of repairs of a unit suffice for a stochastical increase of time to system failure. Certain connection between the solution of this problem and a notion of the convenience of the state-dependent preventive maintenance is shown.