r.eventMigration(...)
Overview
Section titled “Overview”r.eventMigration(eventName, fromVersion, toVersion, transform) —
registers a step-wise payload upcast for event-schema evolution.
toVersion must be fromVersion + 1; chain larger jumps step by step.
Transforms are pure old-payload-in/new-payload-out functions and run once
per READ, not once per event persisted — keep them cheap.
From packages/framework/src/engine/feature-ast/patterns.ts -> EventMigrationPattern