This paper presents a new approach to assessing Requirements Stability as a contribution to the building of more stable long lifetime systems. A goal-based model is used to analyze the requirements in a number of possible future worlds described by scenarios of the possible future environment of the system. The result is an assessment of possible instability of the requirements and the assumptions, which can be presented to system developers to inform alternative requirements definition and architectural choices supporting "targeted design for change". A process for carrying out the analysis is described and practical tool support outlined. Results from an industrial scale, live case study validating the approach, process and tool support is reported, and possible developments of the concept, process and tool are discussed.