Plan a breaking API version change

Help me plan a breaking API version change.
Inputs: current api; changes; clients.

1. Map contract changes and affected consumers.
2. stage coexistence, compatibility checks and migration.
3. define deprecation communication, monitoring and recovery.

Deliver: Change classification, Avoid the break, Versioning, Compatibility layer, Timeline, Telemetry, Client migration guide.
Clarify critical gaps, separate facts from assumptions, and do not invent data, citations or completed actions.

My material: [fill in here]

Before you start

Provide current api; changes; clients.