  Model evolution is widely considered as a subject under research. Despite its
role in research, common purpose concepts, approaches, solutions, and
methodologies are missing. Limiting the scope to model libraries makes model
evolution and related quality concerns manageable, as we show below. In this
paper, we put forward our quality staged model evolution theory for model
libraries. It is founded on evolution graphs, which offer a structure for model
evolution in model libraries through evolution steps. These evolution steps
eventually form a sequence, which can be partitioned into stages by quality
gates. Each quality gate is defined by a lightweight quality model and
respective characteristics fostering reusability.
