In the paper, a generalized, based on a practical experience, approach to the organization of the technical maintenance and modernization of elaborate soft- and hardware complexes of APCS/SCADA for large-scale power engineering plants, involving nuclear power plants (NPP). An analysis of features and risks under the modernization is implementing. The application domain of the approach may cover plant of power engineering, chemistry, oil industry, and other large-scale plants, where complex integrated APCS are applied, especially, within continuous operation modes.