AWS shipped zero-downtime migration into ElastiCache Serverless for Valkey. Under the hood it is PSYNC plus a proxy that flips per shard. We ship PSYNC without the proxy. Here is what each one gives up, and the two things I'm fixing in ours because their docs made me notice.
Redis re-added an OSI-approved license in May 2025, so why move off KeyDB to Valkey instead of Redis? Because 'open source again' is a license one company added and can route around next time, Valkey's structure has no mechanism to re-close, your RESP clients already work today, and the native-client story runs on AWS's and Google's payroll.
BetterDB's migration tooling gives you a complete picture before you move a single key -- and confirms the picture after.