Remote | Senior Software Engineer (Legacy Codebase Migration) — Up to $180/hour

24-Mag•New York, NY

13d•$180•Remote

About The Position

We are sharing a specialised consulting opportunity for experienced Software Engineers with deep expertise in legacy codebase migration, large-scale systems modernisation, and complex production-grade software environments. This opportunity is designed for engineers who have personally executed substantial migrations across mature, mission-critical systems. The work involves designing, implementing, reviewing, and evaluating complex modernisation tasks while preserving functionality, compatibility, data integrity, security, performance, and production reliability.

Requirements

5+ years of professional software engineering experience, ideally at Senior, Staff, Principal, Lead, or equivalent level.
Significant hands-on experience executing large-scale production migrations.
Personally owned or made substantial technical contributions to complex modernisation initiatives.
Strong experience working with mature systems involving technical debt, complex dependencies, legacy integrations, and significant regression risk.
Strong knowledge of compatibility, testing, observability, staged rollouts, rollback strategies, and behavioural equivalence.
Ability to reason deeply about unfamiliar codebases and diagnose subtle migration failures.
Experience with one or more major migration categories including languages, frameworks, databases, APIs, infrastructure, CI/CD, authentication, repositories, testing, or dependencies.

Nice To Haves

Particularly valuable experience modernising systems within financial services, fintech/payments, trading, insurance, healthcare, telecom, large technology companies, or other high-reliability environments.
For mainframe modernisation work, experience with COBOL, z/OS, CICS, JCL, DB2, VSAM, batch processing, and COBOL-to-Java/Python migration is particularly valuable.

Responsibilities

Legacy Codebase Migration & Modernisation: Design, implement, review, and evaluate complex codebase migration and modernisation tasks.
Analyse mature production systems to determine safe and technically sound migration approaches.
Review legacy application architecture, dependencies, APIs, databases, build systems, and infrastructure.
Identify technical debt, compatibility constraints, migration risks, and hidden dependencies.
Develop migration strategies appropriate for large, interconnected production environments.
Language & Runtime Modernisation: Evaluate migrations involving major language or runtime upgrades.
Work with scenarios such as Java 8 to Java 17/21, legacy .NET to modern .NET, Python 2 to Python 3, and PHP 7 to PHP 8.
Review language rewrites such as COBOL to Java/Python and JavaScript to TypeScript.
Assess behavioural equivalence and compatibility across legacy and modern implementations.
Identify regressions introduced through language, runtime, or compiler changes.
Framework & Application Migration: Review major upgrades or replacements involving frameworks such as Angular, Rails, Django, Spring/Spring Boot, ASP.NET, Next.js, and Astro.
Assess architectural implications of replacing or substantially upgrading mature application frameworks.
Evaluate dependency compatibility and integration risks.
Review implementation approaches for maintainability and production readiness.
Identify subtle behavioural differences introduced during framework migrations.
Database & Data-Layer Modernisation: Evaluate migrations such as Oracle/MySQL to PostgreSQL and SQL to NoSQL.
Review ORM and data-access transitions including Hibernate to jOOQ, Sequelize to Prisma, and raw SQL to ORM.
Assess data integrity, schema compatibility, migration sequencing, and application dependencies.
Identify risks involving data loss, behavioural changes, transaction semantics, and performance.
Review validation approaches for complex production data migrations.
Infrastructure & Cloud Modernisation: Review transitions from VMs or bare metal to containerised and cloud-native architectures.
Evaluate migrations involving Docker, Kubernetes, and modern infrastructure patterns.
Assess deployment architecture, operational dependencies, scalability, and production resilience.
Review staged rollout and rollback strategies.
Evaluate whether infrastructure changes preserve reliability and operational visibility.
CI/CD & Build Tooling: Review modernisation of deployment pipelines such as Jenkins/Travis to GitHub Actions or GitLab CI.
Evaluate build and package-tool migrations including Maven to Gradle, Webpack to Vite, CRA to Next.js, and npm to pnpm.
Assess build reproducibility, dependency management, deployment safety, and automation quality.
Identify migration risks within mature CI/CD environments.
Review changes for maintainability and operational effectiveness.
API & Integration Migration: Evaluate transitions such as REST to GraphQL, HTTP/REST to gRPC, and SOAP or legacy APIs to modern interfaces.
Assess backward compatibility and downstream integration impact.
Review API contracts, dependency changes, and migration sequencing.
Identify compatibility risks and behavioural differences.
Evaluate strategies for maintaining continuity during interface transitions.
Authentication & Identity Modernisation: Review migrations from legacy authentication systems to OAuth/OIDC, SAML/SSO, and modern identity platforms.
Assess security, compatibility, authorisation, and integration implications.
Identify authentication or access-control regressions.
Review migration sequencing and transition strategies.
Evaluate implementation quality and production readiness.
Repository, Dependency & Toolchain Migration: Evaluate polyrepo to monorepo transitions using tools such as Bazel, Nx, or Turborepo.
Review replacement of deprecated or end-of-life libraries, APIs, runtimes, and vendor SDKs.
Assess dependency graphs and compatibility across large software ecosystems.
Identify upgrade risks created by transitive dependencies or legacy integrations.
Evaluate migration strategies for maintainability and long-term sustainability.
Testing, Validation & Production Safety: Develop and review testing strategies for complex migrations.
Evaluate unit, integration, browser, and end-to-end testing migrations.
Identify migration regressions and behavioural differences.
Assess compatibility, observability, staged rollout, and rollback strategies.
Evaluate whether proposed migration solutions are sufficiently validated for production use.
Technical Evaluation: Review migration solutions for correctness, completeness, maintainability, and production readiness.
Analyse unfamiliar codebases and diagnose subtle migration failures.
Evaluate proposed technical decisions against realistic enterprise constraints.
Identify missing dependencies, edge cases, regression risks, and unsafe assumptions.
Provide clear, technically rigorous feedback on migration approaches and implementations.