To address the concern with changing "immutable" events. I've just built a migrator [1] tool for my PostgreSQL-based event store. It implements the copy & transform migration strategy. The source event store database is untouched. Transformed events are written out to a separate database.

It uses Elixir streams to provide composable transforms to rewrite, aggregate, remove, and alter serialization format of events.

[1] https://github.com/slashdotdash/eventstore-migrator