The evolution of software systems often produces incompatibilities with existing data and applications. To prevent incompatibilities, changes have to be well-planned, and developers should know the impact of changes on a software system. This consideration also applies to the field of model-driven development, where changes occur with the modification of the underlying metamodels. Models that are instantiated from an earlier metamodel version may not be valid instances of the new version of a metamodel. In contrast to other metamodeling standards like the Eclipse Modeling Framework (EMF), no classification of metamodel changes has been performed yet for the Meta Object Facility (MOF). The contribution of this paper is the evaluation of the impact of metamodel changes on models. For the formalisation of changes to MOF-based metamodels, a Change Metamodel is introduced to describe the transformation of one version of a metamodel to another. The changes are then classifed by their impact on the compatibility to existing model data. The classification is formalised using OCL constraints. The Change Metamodel and the change classifications presented in this paper lay the foundation for the implemention of a mechanism that allows metamodel editors to estimate the impact of metamodel changes semi-automatically.