Ok, this might not cover EVERYTHING, but starting with your actual databases - implement comprehensive backups for all your data, ensuring they are consistent and tested for restoration. Use database replication and snapshots where possible. Make sure also that for your microservices are stateless and the data is backed up somewhere in a cache, that itself has backups. Apply strict version control and schema migration tools to your databases, and ensure data integrity with thorough regular testing. Get an observability platform to monitor logs and metrics to detect issues early, and always have a rollback plan in case of failure. Lastly ensure that throughout all of this - that the data is securely encrypted and transmitted.

During massive upgrades, protecting data is priority number one. My go-to strategy starts with comprehensive backups, ideally with versioned copies stored in both on-prem and cloud environments for redundancy. I also test the upgrade process in a sandbox environment first, running a small-scale simulation to identify any data corruption risks before going live. Additionally, implementing transaction logging ensures that even if something goes wrong mid-upgrade, we can roll back to a safe state. Finally, I always have a fail-safe plan—a rollback procedure that can revert the system to its pre-upgrade condition if needed, ensuring no critical data is lost.