Modernising a business-critical system without stopping the business
A repeatable, fully tested move of a 13-year-old system onto a modern platform, delivered alongside new features by developers working with an AI assistant.
- ~2,000code files converted automatically
- 2,827tests passing on old and new
- 1,000+changes reviewed in under 7 months
The client
A UK online retailer whose back-office system has been in daily use for over a decade.
The challenge
The system had grown for thirteen years on technology that is no longer being developed. A rewrite was out of the question: it runs the warehouse, purchasing and finance every day, and peak season can't be put at risk. Modernisation had to happen alongside ongoing work, with proof at every step that nothing had changed for the people using it.
What we built
- A one-step conversion that can regenerate the whole modern version from the current system at any time, so day-to-day work never stops
- Automatic checks on every conversion: it must build, pass the same tests as the original, and produce compiled code that matches the original's
- A side-by-side copy on the modern platform that staff can use for real work, with a screen-by-screen visual comparison
- A repeatable upgrade tool for the next step onto the latest Microsoft platform
- An AI assistant that works as part of the team: it researches each request, asks clarifying questions, plans and makes the change, and passes it to a developer for review
The result
The whole system now converts automatically, builds cleanly and passes the same 2,827 tests as the original. Staff have been able to use the converted version alongside the live one since September 2026.
Technical details
VB.NET is converted to C# with a customised open-source converter inside a harness that decompiles both versions and compares them. Views go through a separate converter checked by the ASP.NET compiler, and pixel comparisons check live against converted pages. Fixes always go into the tooling, never into the output, so the conversion can be reproduced exactly at any time.
A deterministic .NET 10 upgrade tool handles the framework move. The AI assistant is event-driven from Azure DevOps, and every change it makes goes through pull-request review and automated builds.
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