Incremental migration works only when the exit strategy is as explicit as the first slice. Otherwise the new system becomes another permanent layer beside the old one.
Define ownership boundaries
Each migrated capability needs one source of truth, one operational owner, and one rollback path. Shared ownership feels cooperative until production incidents arrive.
Move traffic with evidence
We use shadow reads, dual-write audits, and customer-safe cohorts before moving critical paths. The important metric is not percentage migrated, but risk retired.
Delete the old path
The migration is not complete when the new feature works. It is complete when the obsolete code, dashboards, alerts, and process debt are gone.