An unanswered question in information systems (IS) research is: What triggers organizations to undertake reengineering or replacement of a mission-critical information system? While the benefits of reengineering efforts and new system implementations are well-documented, these large scale discontinuous system-changes are costly and known to be risky. IS projects frequently fail. A great deal of research has focused on understanding how new system implementations can be successful, once the decision to move forward is made. Alternatively, this research examines the antecedents of undertaking that costly and risky discontinuous change, as well as the role of inertia that precipitates the undertaking. We conducted 37 semistructured interviews in three organizations. We explored the reasons for replacing or reengineering their mission-critical information systems. We employ three theoretical explanations of change as derived from the organization theory and strategy literatures to conduct a cross-case analysis. While in some case studies external environmental forces and internal strategy changes reveal themselves as antecedents, we found that across all three cases, inherent constraints in the system slowed and even stifled the adaptation and design of these systems. We draw on structural inertia organizational change theories to explain these antecedents and how the systems evolved into their current states.