What happens when you have half a million users, 11 millions of their files, and you have to change completely the underlying technology with no downtime, no conflicts, over several weeks of a rolling update? Arduino Create is a web application that handles half a million users, 11 millions of files totaling 100GB. Files were saved in AWS S3 but we decided that using a NFS shared between instances was easier and better performing. But how to migrate data without stopping the service for several hours? How to do a rolling update, in order to minimize issues ? In this talk we examine the various solutions considered and focus on the final one, that involves using interfaces and a synchronization mechanism.