Centralized Version Control Systems have been used by many open source projects for a long time. However, in recent years several widely-known projects have migrated their repositories to Distributed Version Control Systems, such as Mercurial, Bazaar, and Git. Such systems have technical features that allow contributors to work in new ways, as various different workflows are possible. We plan to study this migration process to assess how developers' organization and their contributions are affected. As a first step, we present an analysis of the Mozilla repositories, which migrated from CVS to Mercurial in 2007. This analysis reveals both expected and unexpected aspects of the contributors' activities.