Service-oriented architecture is a recent paradigm for architectural design. The software engineering aspects in this context, that have not been sufficiently addressed, are software evolution and software migration. Architectures are of great importance if large software systems change. Architectural transformations can guide and make this change controllable. In this paper, we present a modelling and transformation method for service-based software systems. Architectural configurations, expressed through architectural patterns, form the core of an underlying specification and transformation calculus. Patterns on different levels of abstraction form transformation invariants that structure and constrain the transformation process. We explore the role layered patterns can play in modelling and as invariants for transformation techniques.