In this talk I am going to present possible approaches for a Python 3 migration of existing Python code running on Python 2.7. I'll evaluate which of these approaches seems suitable for a large code base and which principles should be paid attention to to increase the probability of a successful migration project. I'll use union.cms (a content management system used by German trade unions) to show the story of a successful migration project involving nearly one million lines of Python code. The story will include the obstacles we encountered and what we learned on the way. I believe that most of the discussed ideas and experience can also be used unchanged in smaller projects as the presented principles are very much the same. Goal of the presentation: You should be able to come up with a plan for migrating your Python 2 based projects to Python 3.