Today?s world is one of rapid and constant change. Computer-based information systems, as integral components of our businesses and personal lives, must change and adapt to maintain their effectiveness. IS professionals face the challenges of analyzing, enabling, and controlling information systems changes. In this paper, we present a formal model for viewing and analyzing information systems change. Drawing upon the biological model of punctuated equilibrium, we identify equilibriums in the IS life cycle at which system metrics can be collected and compared with previous equilibriums. Based upon one or more system metrics, a set of COR measures are developed to study system core, obsolescence, and recency. We demonstrate the application of the COR model to two interesting cases of information systems change. Conclusions and future research directions complete the paper.