Changing requirements for health care information systems force the development of an open, modular architecture in which components can be integrated. This offers a flexible means for integrating different (heterogeneous) systems used by different users. Migration towards such an open, modular architecture is a difficult task. It consists of breaking down existing systems in components and integrating these components. Many authors on migration strategies focus attention on the ordering of the steps to be taken without detailing how these steps were arrived at. This paper presents a more rigorous approach for deriving a migration strategy.