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

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.

Key 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 Java 17/21, legacy .NET modern .NET, Python 2 Python 3, and PHP 7 PHP 8
• Review language rewrites such as COBOL Java/Python and JavaScript 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 PostgreSQL and SQL NoSQL
• Review ORM and data-access transitions including Hibernate jOOQ, Sequelize Prisma, and raw SQL 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 GitHub Actions or GitLab CI
• Evaluate build and package-tool migrations including Maven Gradle, Webpack Vite, CRA Next.js, and npm 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 GraphQL, HTTP/REST gRPC, and SOAP or legacy APIs 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 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

Ideal Profile

• 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
• 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

Engagement Details

• Independent contractor engagement
• Fully remote
• Flexible scheduling based on project requirements
• Talent-network opportunity with potential for ongoing project work
• Work focuses on technically challenging migration problems modelled after large-scale production systems
• Projects may be extended, shortened, or concluded based on project needs and performance
• Work must be completed without using confidential or proprietary information belonging to any employer, client, institution, or other third party
• H1-B and STEM OPT support is unavailable for this engagement

About the Platform

This opportunity is available through 24-MAG LLC. We connect experienced professionals with remote consulting opportunities across technical, evaluation, and project-based workstreams.

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