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Legacy Modernisation

Modernise the system without stopping the business.

The system everyone complains about is usually the one nothing can be switched off without. It holds twelve years of data, three undocumented integrations and the only copy of a business rule nobody can now explain. Replacing it is an archaeology problem before it is an engineering one.

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In one paragraph

AGI Software Solutions modernises ageing business systems incrementally rather than through a rewrite-and-switch: we map what the system does and who depends on it, move one workflow at a time behind a stable interface, and run old and new in parallel until the numbers reconcile. Modernisation programmes typically run $40k–$90k for a single application and $100k–$250k+ for a core system with data migration and multiple dependent integrations.

What We Build

Rewrites fail. Migrations succeed.

The full rewrite is the most reliably disastrous pattern in enterprise software: two years of parallel effort, a feature freeze on the system people actually use, and a cutover date that slips until the project is cancelled. Incremental replacement avoids all three.

Continuity

The business keeps running

No feature freeze and no cutover weekend. The old system keeps serving until each replacement has proven itself against it in production.

Archaeology

The undocumented rules, recovered

We read the code and the data to recover the business logic nobody wrote down, and confirm it with the people who rely on it before anything is reimplemented.

Reversibility

Every step can be undone

Each migrated workflow can be routed back to the legacy system while it settles. Nothing depends on a single irreversible decision.

Reconciliation

Data verified, not assumed

Old and new run in parallel with automated comparison, so migration finishes when the outputs agree — not when the calendar says so.

Plain English

What does legacy modernisation actually involve?

Legacy modernisation is the work of moving a business off a system that still works but has become expensive to change — because the framework is unsupported, the original team has gone, the hosting is end-of-life, or every small change now takes weeks. It covers replatforming, re-architecting, data migration and, often, rebuilding the parts nobody can safely touch.

The technique that makes it survivable is the strangler pattern: put a stable interface in front of the old system, move one capability at a time behind it, and let the legacy system shrink until switching it off is a non-event rather than a launch.

Compared

Full rewrite vs incremental migration

We have been asked for the left column many times. We have recommended it approximately never.

Full rewriteIncremental migration
First value deliveredAt cutover, if it arrivesWithin weeks, workflow by workflow
Risk profileOne enormous switchoverMany small reversible steps
The old systemFrozen while you rebuildKeeps running and keeps improving
If priorities changeSunk cost, nothing shippedEverything migrated so far still works
Data migrationOne high-stakes eventContinuous reconciliation, verified as you go
Honest success ratePoor, and well documentedGood, because failure stays small
How We Build It

Five stages from problem to production.

The same process on every engineering project, scaled to the size of the problem.

01

Discovery

We map what the process actually does today, including the parts that live in one person’s head, and what it costs when it goes wrong.

02

Architecture

Data model, integration points, permissions and hosting decided up front, because these are the choices that are expensive to reverse later.

03

Build in slices

Working software every couple of weeks, sequenced so the riskiest piece reaches real users first rather than last.

04

Integrate and harden

Connecting to the systems that hold your data, then security, load and failure testing against realistic volumes instead of demo data.

05

Handover and support

Documentation, a walkthrough for whoever maintains it, and ongoing support if you want it. The repository is yours either way.

Use Cases

Where legacy modernisation pays off.

The patterns we see deliver, across startups, SMEs and enterprise teams.

Replatforming

Off end-of-life technology

Applications on unsupported frameworks, database versions past their support window, or servers nobody wants to be responsible for patching.

Re-architecting

Breaking up the monolith

Extracting the parts that need to scale or change often, without the fashionable mistake of shattering a working system into forty services.

Data

Migration and reconciliation

Moving years of records with the schema changes, deduplication and verification that make the new numbers trustworthy.

Interfaces

APIs over systems without them

Wrapping a closed system in a clean API so new tools — including AI — can reach the data without touching the legacy code.

Tech Stack

What we build on.

Deliberately boring, widely supported technology. The goal is software your team can still hire for in five years.

Front end

React, Next.js and TypeScript, with server rendering where it earns its keep for speed or SEO.

Back end

FastAPI, Django and Node.js. Python where the work touches data or models, TypeScript where it does not.

Data

PostgreSQL as the default, Redis for caching and queues, and a vector store only when retrieval genuinely needs one.

Infrastructure

Docker and Kubernetes on AWS, GCP, Azure or your own servers. Your cloud account, your bill, your control.

Integration

REST and GraphQL APIs, webhooks, message queues and MCP servers for the systems that need to talk to AI.

Quality

Automated tests, CI on every commit, staged deploys and monitoring wired in before launch rather than after the first outage.

Related Work

Systems we have shipped.

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What Teams Say

Hear from the teams we work with.

“We needed a voice agent that could actually qualify leads, not a chatbot pretending to be one. The team shipped a sub-700ms pipeline in 6 weeks. It now handles 5k calls a day.”
Priya RajHead of Growth, Ninjatech
“What sold us was their willingness to put AI engineers and product designers on the same call. We got working prototypes by week two and a production rollout in three months.”
James ThorntonCTO, Allindex
“AGI designed a CRM system tailored to our client management process. It is intuitive, reliable, and has centralized all our communication and history in one dashboard. This has greatly improved client retention.”
Carlos MendesProduct Manager, Qilinlab
Common Questions

Before you get in touch.

How much does legacy modernisation cost?

A single application replatformed or rebuilt typically runs $40k–$90k. A core business system with substantial data migration and several dependent integrations runs $100k–$250k and up. The dominant cost is almost always understanding and migrating the data, not writing the new code.

Can you modernise without downtime?

In most cases, yes. Workflows move one at a time behind a routing layer, with the legacy system still serving everything not yet migrated. Brief planned windows are sometimes needed for a final data cutover, and we schedule those with you rather than around you.

Nobody here understands how the old system works. Is that a problem?

It is the normal starting position. We recover behaviour from the code, the database and observed traffic, then confirm the reconstructed rules with the people who depend on the outputs. That discovery phase is real work and we scope it explicitly rather than pretending it is free.

Should we move to microservices?

Only where a specific component needs to scale or change independently. Splitting a working monolith into many services usually converts one manageable problem into a distributed one. We extract what benefits from extraction and leave the rest alone.

Can we add AI while we modernise?

That is often the trigger for the project. Legacy systems block AI because the data cannot be reached; putting a clean API over it during migration is what unblocks document processing, search and assistants later. We would still sequence the migration first.

What if we stop halfway?

Everything migrated so far keeps working, and the legacy system carries the rest. That is the point of incremental migration: pausing costs you the remaining benefit, not the investment already made.

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Talk to us about legacy modernisation.

Tell us what the system would need to do and what it is replacing. We will tell you whether it is worth building and roughly what it takes.

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