OpenAI Select PartnerLegacy Software Modernisation: How to Replace Old Systems Without Stopping the Business
Old software often runs critical parts of a business: billing, inventory, operations. Replacing it all at once is risky. The safer path is usually to modernise in stages, keeping the business running while each piece moves to the new system.
In this guide 9 sections
Some of the most rewarding projects we have done replaced spreadsheets and old systems that a business had outgrown, such as our manufacturing ERP. The lesson each time was the same: move in stages, and never switch everything off on one day.
The principle
Replace the old system one piece at a time, starting where the pain is greatest, and run old and new side by side until the new one is trusted.
Signs it is time
- Small changes take weeks and often break something else.
- The technology is no longer supported or receives no security updates.
- Only one or two people understand how it works.
- It cannot connect to modern tools, apps or APIs.
- Teams keep side spreadsheets because the system cannot show what they need.
- Hosting or licence costs keep rising.
The main options
| Option | What it means | When it fits |
|---|---|---|
| Rehost | Move to the cloud as it is | The code is fine; the hosting is the problem |
| Refactor | Improve the code without changing what it does | The design is sound but the code is messy |
| Rebuild in stages | Replace modules one by one | The system blocks the business but cannot stop |
| Replace | Move to an off-the-shelf product | The process is standard and a product fits |
The strangler approach
Described by Martin Fowler as the strangler fig application, this approach puts new modules around the old system and moves users across one area at a time. Each step delivers value, and if something goes wrong only one area is affected.
Living with an old system?
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- Inventory every table, file and spreadsheet the old system depends on.
- Map old fields to the new structure and decide what to leave behind.
- Clean duplicates and inconsistent records before moving them.
- Run test migrations and check samples with the people who use the data.
- Keep the old system read-only for a period after cutover.
A staged cutover plan
- Discover: map processes, users, data and integrations.
- Prioritise: pick the module with the most pain and least risk.
- Build and test: including integrations with the parts still on the old system.
- Run in parallel: compare results between old and new.
- Train and switch: move one team or location at a time.
- Repeat for the next module.
The same discipline is behind our article on why custom ERP implementations fail.
Moving to the cloud
Modernisation is often a good moment to move hosting to the cloud, with backups, monitoring and security updates handled properly. See AWS vs Azure vs Google Cloud and our cloud consulting service.
Common mistakes
- A "big bang" switch-over on a single weekend.
- Copying every old screen and workaround into the new system.
- Leaving data migration until the end.
- Not involving the people who use the system every day.
How we modernise
At Creuto we modernise legacy systems in stages, with parallel running and careful data migration, so operations continue throughout. After that, a proper maintenance plan keeps the new system healthy.
Frequently asked questions
What is legacy software?
Software that is still important to the business but is hard to change, built on outdated or unsupported technology, or understood by only a few people.
Should we rewrite from scratch?
Rarely all at once. A full rewrite delays value and carries high risk. Replacing the system in stages, starting with the most painful part, is usually safer.
What is the strangler fig approach?
A pattern where new functionality is built around the old system piece by piece, with traffic gradually routed to the new parts, until the old system can be switched off.
How do we move the data safely?
Map the old data to the new structure, clean it, run test migrations, check results with the people who use the data, and keep the old system available read-only for a period after cutover.
How long does modernisation take?
It depends on the size of the system. A staged approach delivers the first improved module much sooner than a full rewrite would, and the rest follows in planned phases.
Ready to modernise?
We modernise legacy applications in stages, with data migration and parallel running so operations never stop.
About Nihar Ranjan Rout
Founder & CEO of Creuto. Over the past 8+ years Nihar has led product strategy and engineering for 50+ products, including mobile apps, custom ERPs and B2B platforms. Creuto is an OpenAI Select Partner. He works directly with founders to turn an idea into a clear plan and a launched product.
More about Nihar · LinkedIn