Legacy Migration Plan

Plan a migration from {{old_system}} to {{new_system}}. Size and complexity: {{size}}. Constraints (uptime, team, deadline): {{constraints}}. Integrations: {{integrations}}.

Provide: an inventory approach; the strategy (strangler, parallel run, big bang — recommend one with reasons); phases with exit criteria; data migration and verification approach; testing strategy including characterization tests; feature flags and rollback for each phase; risks and mitigations; communication plan; and a first two-week plan. Highlight the unknowns I should discover with a spike first.