Listen to the data that you're migrating from one system to another, so to speak. Test v1-to-v2 and v2-to-v1 migrations until you're blue in the face. Feature-flag migrations for individual clients. Ensure that any SLAs are met with v1-only, v1-in-flight-to-v2, v2 only, and/or some mix of static partial migration. Make sure that you have an absolutely homeomorphic mapping of data from one representation to another.