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

Note: The job is a remote job and is open to candidates in USA. 24-MAG LLC connects experienced professionals with remote consulting opportunities across technical, evaluation, and project-based workstreams. The Senior Software Engineer will design, review, evaluate, and implement complex legacy codebase migrations and large-scale systems modernisation while preserving functionality, compatibility, data integrity, security, performance, and production reliability. The role also assesses language, framework, database, infrastructure, CI/CD, API, authentication, dependency, testing, and production-readiness risks. Responsibilities Design, implement, review, and evaluate complex codebase migration and modernisation tasksAnalyse mature production systems to determine safe and technically sound migration approachesReview legacy application architecture, dependencies, APIs, databases, build systems, and infrastructureIdentify technical debt, compatibility constraints, migration risks, and hidden dependenciesDevelop migration strategies appropriate for large, interconnected production environmentsEvaluate migrations involving major language or runtime upgradesWork with scenarios such as Java 8 Java 17/21, legacy .NET modern .NET, Python 2 Python 3, and PHP 7 PHP 8Review language rewrites such as COBOL Java/Python and JavaScript TypeScriptAssess behavioural equivalence and compatibility across legacy and modern implementationsIdentify regressions introduced through language, runtime, or compiler changesReview major upgrades or replacements involving frameworks such as Angular, Rails, Django, Spring/Spring Boot, ASP.NET, Next.js, and AstroAssess architectural implications of replacing or substantially upgrading mature application frameworksEvaluate dependency compatibility and integration risksReview implementation approaches for maintainability and production readinessIdentify subtle behavioural differences introduced during framework migrationsEvaluate migrations such as Oracle/MySQL PostgreSQL and SQL NoSQLReview ORM and data-access transitions including Hibernate jOOQ, Sequelize Prisma, and raw SQL ORMAssess data integrity, schema compatibility, migration sequencing, and application dependenciesIdentify risks involving data loss, behavioural changes, transaction semantics, and performanceReview validation approaches for complex production data migrationsReview transitions from VMs or bare metal to containerised and cloud-native architecturesEvaluate migrations involving Docker, Kubernetes, and modern infrastructure patternsAssess deployment architecture, operational dependencies, scalability, and production resilienceReview staged rollout and rollback strategiesEvaluate whether infrastructure changes preserve reliability and operational visibilityReview modernisation of deployment pipelines such as Jenkins/Travis GitHub Actions or GitLab CIEvaluate build and package-tool migrations including Maven Gradle, Webpack Vite, CRA Next.js, and npm pnpmAssess build reproducibility, dependency management, deployment safety, and automation qualityIdentify migration risks within mature CI/CD environmentsReview changes for maintainability and operational effectivenessEvaluate transitions such as REST GraphQL, HTTP/REST gRPC, and SOAP or legacy APIs modern interfacesAssess backward compatibility and downstream integration impactReview API contracts, dependency changes, and migration sequencingIdentify compatibility risks and behavioural differencesEvaluate strategies for maintaining continuity during interface transitionsReview migrations from legacy authentication systems to OAuth/OIDC, SAML/SSO, and modern identity platformsAssess security, compatibility, authorisation, and integration implicationsIdentify authentication or access-control regressionsReview migration sequencing and transition strategiesEvaluate implementation quality and production readinessEvaluate polyrepo monorepo transitions using tools such as Bazel, Nx, or TurborepoReview replacement of deprecated or end-of-life libraries, APIs, runtimes, and vendor SDKsAssess dependency graphs and compatibility across large software ecosystemsIdentify upgrade risks created by transitive dependencies or legacy integrationsEvaluate migration strategies for maintainability and long-term sustainabilityDevelop and review testing strategies for complex migrationsEvaluate unit, integration, browser, and end-to-end testing migrationsIdentify migration regressions and behavioural differencesAssess compatibility, observability, staged rollout, and rollback strategiesEvaluate whether proposed migration solutions are sufficiently validated for production useReview migration solutions for correctness, completeness, maintainability, and production readinessAnalyse unfamiliar codebases and diagnose subtle migration failuresEvaluate proposed technical decisions against realistic enterprise constraintsIdentify missing dependencies, edge cases, regression risks, and unsafe assumptionsProvide clear, technically rigorous feedback on migration approaches and implementations Skills 5+ years of professional software engineering experience, ideally at Senior, Staff, Principal, Lead, or equivalent levelSignificant hands-on experience executing large-scale production migrationsPersonally owned or made substantial technical contributions to complex modernisation initiativesStrong experience working with mature systems involving technical debt, complex dependencies, legacy integrations, and significant regression riskStrong knowledge of compatibility, testing, observability, staged rollouts, rollback strategies, and behavioural equivalenceAbility to reason deeply about unfamiliar codebases and diagnose subtle migration failuresExperience with one or more major migration categories including languages, frameworks, databases, APIs, infrastructure, CI/CD, authentication, repositories, testing, or dependenciesFor mainframe modernisation work, experience with COBOL, z/OS, CICS, JCL, DB2, VSAM, batch processing, and COBOL-to-Java/Python migration is particularly valuableParticularly valuable experience modernising systems within financial services, fintech/payments, trading, insurance, healthcare, telecom, large technology companies, or other high-reliability environments Benefits Fully remoteFlexible scheduling based on project requirementsPotential for ongoing project work Company Overview At 24-MAG, we support emerging AI and consulting platforms by sourcing and connecting qualified professionals with remote, contract-based opportunities. It was founded in undefined, and is headquartered in Sheridan, Wyoming, US, with a workforce of 2-10 employees. Its website is https://24-mag.com/.