Open Heart Surgery with 60 Terabytes of Data: Migrating 40.000 users from PowerFolder to Nextcloud

<!--HTML-->tl;dr: It works! System migrations are rarely easy. Neither do you want to lose any data (worst case) or metadata (close to the above) along the way, nor - and that might be the worst case in the long run - do you want to lose the trust of the end users after having decided to go for a new solution. When migrating 40.000 users from 55 universities and higher education organizations with 60 TB of data, from PowerFolder to Nextcloud, you want to make perfectly sure that on Monday morning everybody finds everything to be as complete as it was on Friday afternoon. By the following Thursday you also hope to be able to stop thinking about the word "rollback" coming from the customer to your private Telegram in the middle of the night. Let's look at some learnings from this major migration project of 2020 and what had to be done in terms of logic analysis and custom programming (which ended up in Nextcloud's core, open for everyone) so students, teachers, staff and guests can work effectively and efficiently with a top of class content collaboration platform.