Software evolution is characterized by inevitable changes of software and increasing software complexities, which in turn may lead to huge costs unless rigorously taking into account change accommodations. This is in particular true for long-lived systems in which changes go beyond maintainability. For such systems, there is a need to address evolvability explicitly during the entire lifecycle. Nevertheless, there is a lack of a model that can be used for analyzing, evaluating and comparing software systems in terms of evolvability. In this paper, we describe the initial establishment of an evolvability model as a framework for analysis of software evolvability. We motivate and exemplify the model through an industrial case study of a software-intensive automation system.