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.
By submitting this application, you acknowledge that your information may be processed by 24-MAG LLC for recruitment and opportunity matching in accordance with our Privacy Policy: https://www.24-mag.com/privacy-policy.