The Business Process Execution Language (BPEL) is a well established language for the definition of process models as service orchestrations. Service orchestrations are used in conjunction with service choreographies in order to create distributed, complex service-based applications. An important requirement for such applications is the need for flexibility during both their modeling and their execution. This work builds on this need by proposing an extension of BPEL in order to allow the definition of abstract constructs on the level of executable process models. Such constructs can be refined to concrete activities at any time, enabling the reuse of existing models and the dynamic adaptation to changing requirements. The design and implementation of the language extension, as well as that of the supporting environment required for the modeling, provisioning, and execution of such process models is further discussed. A case study on a city-wide public transportation system offers the means for an evaluation of the proposed approach.