Refactoring of UML class diagrams is an emerging research topic and heavily inspired by refactoring of program code written in object-oriented implementation languages. Current class diagram refactoring techniques concentrate on the diagrammatic part but neglect OCL constraints that might become syntactically incorrect by changing the underlying class diagram. This paper formalizes the most important refactoring rules for class diagrams and classifies them with respect to their impact on annotated OCL constraints. For refactoring rules, whose application on class diagrams could make attached OCL constraints incorrect, we formally describe how the OCL constraints have to be refactored to preserve their syntactical correctness. Our refactoring rules are defined in the graph-grammar based formalism proposed by the QVT Merge Group for the specification of model transformations.