Software architectures constitute one of the main artefacts of software-intensive system development.
They outline the essential components and interconnections of a software system at a high level of abstraction, ignoring unnecessary details. How to address the evolution of software architectures, however, is still an important topic of current research.
In this article, we use UML~2 as architectural description language notation and formalise it with graph transformation, with a proof-of-concept implemented in the AGG tool. We use this formalisation to express and reason about architectural evolutions that introduce architectural styles.