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Legacy Modernization · Zero Downtime

Modernise your legacy platform without shutting it down

PHP, .NET, Java or older Node.js, rebuilt on a modern stack with parallel run cutover, full test coverage and 60% faster delivery using LLM coding agents. We have replaced production systems for UK staffing, healthcare and telecom platforms without losing a single hour of uptime.

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RG INSYS LLP rebuilds legacy software platforms (PHP, Laravel, .NET, Java, monolithic Node.js) on a modern Node.js, TypeScript, React and PostgreSQL stack hosted on AWS, GCP or Azure. We use parallel run architecture so the legacy system stays live throughout the migration, and LLM coding agents to cut rebuild time by roughly 60%. UK, US and UAE clients in recruitment, telecom, fintech, healthcare and SaaS rely on us for modernisation work that has to happen without disruption.

What we deliver
Discovery audit, written modernization plan, phased rebuild on a modern stack, data migration, parallel run cutover, automated test suite, CI/CD pipeline, runbook and full handover.
Typical timeline
8 to 12 weeks for a focused monolith Phase 1. 4 to 6 months for a full multi module enterprise platform. 30 to 60 day stabilisation period included.
Pricing from
$25,000 fixed price for audit plus Phase 1. $5,000/month for an ongoing modernization retainer. Phase by phase commitments, no lock in.
Stack
Node.js, TypeScript, React, Next.js, NestJS, PostgreSQL, Redis, RabbitMQ, Docker, Kubernetes, Terraform, AWS / GCP / Azure.
Compliance-ready for
GDPR, SOC 2, HIPAA, PCI DSS. UK, EU and US data residency. AES-256 encryption, TLS 1.3, RBAC, immutable audit logs by default.
What's included

A complete modernization, not just a rewrite

🔍

Discovery audit and risk map

Two weeks of structured analysis. Senior engineers, supported by Claude Code, read the legacy codebase end to end, document undocumented business rules, map integrations and rank operational risks. You receive a written audit and a phased modernization plan you can sign off on.

🏛️

Target architecture design

A modern, boring, hire-able stack. Typed Node.js services, React frontend, PostgreSQL with proper indexes, Redis cache, message queues where they earn their keep. Diagrams, ERDs and ADRs are written before any feature code.

🪜

Strangler fig migration plan

Modules move one at a time. The new platform sits in front of, or alongside, the legacy app. Traffic flips per route or per tenant. You can pause, reverse or extend the timeline at any point. No big bang weekend cutovers.

📦

Data migration and reconciliation

Reversible migration scripts, dry runs against production snapshots, and reconciliation reports comparing legacy vs new for every entity. Audit history, transactions and customer records move with full traceability and no silent loss.

🧪

Tests where there were none

Most legacy systems have near zero automated tests. We add Vitest, Jest, Supertest and Playwright as part of the rebuild. Coverage targets are written into the contract, typically 80%+ on the new backend code and full E2E flows on the highest risk journeys.

🛟

Stabilisation and handover

30 to 60 day post cutover support: P1 fixes inside 4 hours, runbooks delivered, your team trained. Optional ongoing modernization retainer for the next phase, or full handover to your in house team. You always own the code.

Our method

How a legacy modernization actually unfolds

01
Audit, weeks 1 to 2

We read the codebase, map the database, interview stakeholders and inventory integrations. Output: a written audit with a risk map, a phased plan and a fixed price quote for Phase 1.

02
Architecture and parallel run setup, weeks 3 to 4

Target architecture document, ERD, deployment topology. New repo scaffolded with linting, CI, observability and a deployable shell. Parallel run infrastructure stood up so legacy and new can coexist.

03
Module by module rebuild, weeks 5 to N

Two week sprints. AI handles boilerplate, models, tests and migrations. Senior engineers own logic and security. Each module is shipped to staging, run in parallel against production, then cut over only when verified.

04
Cutover and stabilisation

Traffic shifts per module on your schedule. 30 to 60 days of close support after final cutover, with P1 incidents resolved inside 4 hours. Runbook handover and engineer training before we step back.

Our tech stack for legacy modernization

The point of a modernization is to replace something fragile with something your team can hire for, patch and extend. We default to mainstream tools that thousands of engineers already know, and that LLM coding agents handle fluently. Where you have an internal Java or .NET shop, we keep migrating you forward inside that ecosystem rather than forcing a stack swap.

Node.js 20 TypeScript NestJS React 18 Next.js PostgreSQL Redis RabbitMQ / SQS Docker Kubernetes / ECS Terraform GitHub Actions Playwright Sentry Datadog
Proof

A representative case study

Recruitment · UK UK workforce solutions company

PHP/Laravel recruitment portal rebuilt on Node.js + React in 8 weeks

A UK staffing platform was running a PHP/Laravel monolith with 10 to 15 second page loads, manual job board posting and incomplete GDPR compliance. We delivered the new Node.js + React platform in 8 weeks with three engineers, automated five job board integrations and stood up a consent aware document workflow, all without a single hour of unplanned downtime. UK agency quotes had been 16 to 24 weeks at $80k to $120k.

<1sPage load (was 10–15s)
8 wksPhase 1 delivery
5Job board integrations
60%Cost vs UK team

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Pricing

Transparent pricing for legacy modernization

From $25,000

Fixed price for the discovery audit plus Phase 1 of the rebuild. Continue phase by phase, or move to a $5,000/month modernization retainer with a senior engineer plus AI tooling.

  • Two week discovery audit, written modernization plan and risk map
  • Parallel run architecture, no production downtime
  • 80%+ test coverage on rebuilt modules, written into the contract
  • 30 to 60 day stabilisation, P1 fixes inside 4 hours
Full pricing & engagement models →

All pricing transparent. No hidden fees. Free 48-hour written estimate.

FAQ

Common questions

Most legacy rebuilds finish in 3 to 6 months. A focused PHP/Laravel monolith with five to ten core modules and a single database typically reaches Phase 1 cutover in 8 to 12 weeks. Larger Java or .NET platforms with multiple databases, ERP integrations and complex permission models take 16 to 24 weeks. We deliver in two week sprints from week three onwards so you see working software well before the final cutover.
No. We always run the new platform alongside the legacy one. Users keep using the old system until each module is verified on the new stack. Cutover is module by module, not big bang. You decide when traffic shifts. We have done dozens of these migrations with zero unplanned downtime, including for systems running 24/7 telecom and recruitment workloads.
The fixed price covers a structured two week discovery audit (existing codebase, database schema, undocumented business rules, integrations and operational risks), a written modernization plan with phased scope, a target architecture document, and the first phase of build delivering one or two priority modules on the new stack with full tests, CI/CD and a parallel run setup. You can stop after Phase 1 with no further commitment, or continue on a phase by phase basis.
We start with a discovery audit where senior engineers, supported by Claude Code, read the codebase end to end and reconstruct the implicit business rules into a written specification. We pair that with stakeholder interviews to surface tribal knowledge. The output is the first written documentation many of our clients have ever had of their own system, which becomes the source of truth for the rebuild.
It depends on the schema. If the existing database is workable, we keep it and modernise the application layer, often the fastest and lowest risk path. If the schema is the source of the performance and maintainability problems, we design a new schema and migrate data with reversible scripts and a parallel run period. We never throw away history. Audit logs, transactions and customer records are migrated with full traceability.
Default target is Node.js with TypeScript on the backend, React or Next.js on the frontend, PostgreSQL with Redis caching, and AWS or your existing cloud. We pick boring, mainstream technology so hiring stays easy and security patches arrive on time. Where you have a strong preference (Java Spring Boot, .NET 8, Go), we adapt; the methodology stays the same.
Yes. We document everything as we go: architecture decisions, ERDs, OpenAPI specs, runbooks and inline code comments. Tests become living documentation. We also run optional handover sessions and pair programming with your engineers in the final two weeks. Many clients keep us on a smaller retainer for 2 to 3 months post cutover, then move to fully internal ownership.
Often that is the right answer. We use the strangler fig pattern: a new service or frontend slowly absorbs functionality from the legacy system module by module. Some clients only need their slow billing engine rebuilt, or their reporting layer split out, while the rest of the platform stays as is. We will tell you honestly when full rebuild is overkill.
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