Lukasz - Thanks for those links. That's very helpful context.
It sounds like there's no explicit user contract about evolving Coder classes in the Java SDK and users might reasonably assume Coders to be stable between SDK versions. Thus, users of the Dataflow or Flink runners might reasonably expect that they can update the Java SDK version used in their pipeline when performing an update.
Based in that understanding, evolving a class like Metadata might not be possible except in a major version bump where it's obvious to users to expect breaking changes and not to expect an "update" operation to work.
It's not clear to me what changing the "name" of a coder would look like or whether that's a tenable solution here. Would that change be able to happen within the SDK itself, or is it something users would need to specify?