Legacy Codebase Migration Expert

What You'll Do5

• Plan and execute complex codebase migrations while reviewing migration output for correctness and completeness.
• Map legacy code, dependencies, APIs, databases, build systems, and infrastructure to determine safe migration paths.
• Maintain functionality, backward compatibility, data integrity, security, and performance throughout each migration.
• Create test suites and validation strategies that catch regressions and behavioral differences.
• Assess AI-generated migration solutions for production readiness, maintainability, and alignment with requirements.

Requirements7

• 5+ years in professional software engineering, with a Senior, Staff, Principal, or equivalent title.
• Ownership of, or major contributions to, large-scale production migrations in mature codebases.
• Hands-on expertise in at least one migration domain: language version upgrades (Java 8 to 17/21, .NET, Python 2 to 3, PHP 7 to 8), language rewrites (COBOL to Java/Python, JavaScript to TypeScript), framework upgrades (Angular, Rails, Django, Spring, ASP.NET, Next.js, Astro), database/ORM shifts (Oracle to PostgreSQL, SQL to NoSQL, Hibernate to jOOQ, Sequelize to Prisma), infrastructure containerization (VM to Docker/Kubernetes), CI/CD modernization (Jenkins to GitHub Actions, GitLab CI), build tooling (Maven to Gradle, Webpack to Vite, CRA to Next.js, npm to pnpm), API migrations (REST to GraphQL, REST to gRPC, SOAP to modern interfaces), authentication modernization (legacy to OAuth/OIDC, SAML/SSO), monorepo consolidation (Bazel, Nx, Turborepo), and testing framework migrations.
• Strong understanding of compatibility, testing, observability, staged rollouts, rollback strategies, and behavioral equivalence.
• Ability to reason through unfamiliar codebases and pinpoint subtle migration failures.
• Experience modernizing mission-critical systems in banking, payments, trading, insurance, healthcare, telecom, or similar sectors.
• For COBOL/mainframe roles: familiarity with z/OS, CICS, JCL, DB2, VSAM, batch processing, and COBOL-to-Java/Python migrations.

Who Should Apply

The candidate who fits this role has personally led or played a major role in large production migrations and can speak to specific challenges like preserving data integrity and handling rollbacks. Someone with only greenfield development or small-scale refactors will not find the work here. A common reason for rejection is failing to show concrete examples of migration ownership, especially in high-reliability domains like banking or telecom. Candidates who cannot articulate how they validate behavioral equivalence or manage regression risk during phased rollouts often score low.

Salary Insight