Retool's migration of their cloud-hosted 4 TB Postgres database from version 9.6 to 13, driven by the impending end-of-life for version 9.6, was accomplished with minimal downtime through a logical replication strategy. This approach involved running parallel databases, allowing updates to be mirrored from the primary to a secondary database, which minimized downtime compared to the traditional dump and restore method. The process was refined through multiple test runs in a staging environment, development of a detailed runbook, and the use of Warp for enhanced configuration and parallel processing. Challenges included handling large tables and managing foreign key constraints, with lessons learned about the importance of testing with representative workloads and leveraging features like Postgres's NOT VALID constraint option. The upgrade was scheduled during low-traffic periods and successfully completed in around 15 minutes, underscoring the value of careful planning and testing.