Legacy system modernisation is a major challenge in digital transformation, especially when organisations depend on long-lived, business-critical systems that are only partly understood. In such context, organisations must define future needs while determining what current systems actually do and which functions to retain, adapt, or replace. Modernisation is therefore not only a technical challenge but also a requirements engineering (RE) problem, shaped by stakeholder perspectives. This study examines how far stakeholder-centric RE can support gap analysis between the system-as-is and the system-to-be in a legacy modernisation context. We conducted an action research study in a multinational energy company undergoing modernisation, applying stakeholder-centric RE practices, including stakeholder identification, semi-structured elicitation interviews, agreement-building using the Delphi method, and prioritisation with the MoSCoW method. The results show that this process was effective in producing requirements stakeholders generally viewed as understandable and correct, but less effective in achieving agreement on how elicited requirements mapped to legacy system functionality. The findings suggest that stakeholder-centric RE is necessary but not sufficient, pointing to the need for uncertainty- aware, iterative, and evidence-based modernisation practices that combine stakeholder perspectives with manual and tool-assisted analysis of legacy systems.