Services Oriented Architectures preach loosely-coupled ser- vices and high–level composition mechanisms, using for example Web Services to define services and Orchestrations to compose them. But or- chestration evolutions imply modification at source code level. This arti- cle shows how the orchestration paradigm itself can be used to support evolution of Web Services Orchestrations through a behavioral merge process. Using the same model to express orchestrations and evolutions, we expose formally and illustrate in this contribution a merging process helping Wsoa administrators to deal with behavioral evolutions.