As part of the Department of Energy and Electric Power Research Institute's hybrid control room project, detailed human factors engineering guidance was developed for designing human-system interfaces that may be affected by introduction of additional digital technology during modernization of nuclear power plants. The guidance addresses several aspects of human-system interaction: information display, interface management, soft controls, alarms, computer-based procedures, computerized operator support systems, communications, and workstation/workplace design. In this paper, the ways in which digital upgrades might affect users' interaction with systems in each of these contexts are briefly described, and the contents of the guidance developed for each of the topics is also described.