Mediators—used in federated information systems—provide a homogeneous read-only access to a set of autonomous information sources. To achieve semantic integration of the heterogeneous data, correspondences between the autonomous schemas are specified manually by experts. Considering the continuous evolution of those typically long-living information systems, schema evolution is an important aspect. New concepts are necessary to adapt correspondences consistently to evolving schcmas. In this paper we propose a formalized schema evolution mechanism for federated information systems based on metamodeling and dynamic logic. We define useful transactions for schema evolution based on three elements: (1) a metamodel for ODMG schemas, (2) a metamodel for model correspondence assertions (MoCAs), and (3) a classification of evolution actions and their specification. For each evolution transaction, semantic preconditions are specified and the impact on the correspondence assertions are described. In this way, we contribute an important step towards a consistent evolution of federated information systems.