Current workflow management systems (WFMSs) are only applicable in a reliable and secure manner, if the business process (BP) to be supported is wellstructured. As ad hoc deviations from preplanned BPs are rather the norm and form a key part of process flexibility, this limits the applicability of today's workflow (WF) technology significantly. In this paper we present a framework for the support of ad hoc structural changes of WFs. Basic to our approach is a conceptual, graph-based WF model which has a formal foundation in its syntax and operational semantics. Based upon this model we have developed a complete and minimal set of change operations which support users in modifying the structure of WFs at runtime, while preserving their correctness and consistency.