Many industries developing complex products based on embedded systems rely on architecting as a key activity. Furthermore, they use product line approaches to find synergies between their products. This means that they use a base platform which is adapted to different products, and the architecture of the product line thus evolves over time. In previous case studies we have seen that these companies often lack a defined process for the evolutionary architecting of these product lines. The contribution of this paper is to present such a process, which matches key characteristics of mature architecting practices. It is also discussed how this process compares to observations in industry.