With the widespread adoption of object-oriented programming, changing the inheritance hierarchy became an inherent part of today's software maintenance activities. Unfortunately, little is known about the "state-of-the-practice" with respect to changing an application's inheritance hierarchy, and consequently we do not know how the change process can be improved. In this paper, we report on a study of the hierarchy changes stored in a versioning system to explore the answers to three research questions: (1) why are hierarchy changes made? (2) what kind of hierarchy changes are made? (3) what is the impact of these changes? Based on the results of this study, we formulate 7 hypotheses which should be investigated further to make conclusive interpretations on how hierarchy changes fit in the actual change process.