Web Services are designed for loosely coupled systems, which means that in many cases it is not possible to synchronously upgrade all peers of a Web Service scenario. Instead, Web Service peers should be able to coexist in different versions. Additionally, older software versions often could benefit from upgrades to the service if they were able to understand it. This paper presents a framework for semantically extensible schemas for Web Service evolution. The core idea of is to use declarative semantics to describe extensions to a service’s vocabulary. These declarative semantics can be used by older software versions to understand the semantics of extensions, thus enabling older software to dynamically adapt to newer versions of the service. As long as declarative semantics are sufficient, older software can benefit from the service’s extension.