Lambda to kappa migration

Your team maintains a lambda architecture: a streaming layer computes approximate real-time metrics (engagement, revenue) and a separate nightly batch layer recomputes the authoritative numbers, with a serving layer merging both. The pain is doubled logic, every metric is implemented twice (streaming + batch), they drift, and a logic change means two PRs and two reconciliations. Leadership wants to move to a kappa-style single-codebase architecture. Design the migration and the target system, covering reprocessing/replay, correctness equivalence, and what you give up.