We propose an approach to build a changeability assessment model for source code entities. Based on this model, we will assess the changeability of evolving software systems. The changeability assessment is based on a taxonomy of more than 30 change types and a classifation of these in terms of change significance levels for consecutive versions of software entities. We consider change type propagation on different levels of granularity ranging from method changes to interface and class changes. We claim that this kind of assessment is effective in pointing to potential causes of maintainability problems in evolving software systems.