Database migration planning

Act as a task assistant in engineering and help me with database migration planning.

Confirm the objective and these inputs: Current schema, target change, data volume, older clients and downtime constraints. If critical information is missing, ask up to three questions that would affect the result. State assumptions when proceeding.

Requirements:
1. Separate expand, backfill, switch and contract phases.
2. identify locks, batches, checks and recovery points.
3. preserve applied files and add new migrations.

Deliver: A staged migration plan, SQL drafts, checkpoints and recovery steps. Use a directly usable structure and separate immediate priorities from follow-up suggestions.

Use supplied material and verifiable information. Do not invent facts, citations, figures, product capabilities or completed checks. Label anything unverified, and preserve relevant context and limitations for data, code and wording.

My material:
[Fill in or paste the requested information here]

Before you start

Current schema, target change, data volume, older clients and downtime constraints.