How to Modernise Legacy Software with AI: A Step-by-Step Guide 2026
Modernising legacy software is the process of updating or rebuilding old software to meet today’s business and customer needs. Rebuilding on a modern cloud-native platform is far more practical than it was a few years ago, because AI tools can now read and explain existing code in hours rather than months. The safest approach is to rebuild one module at a time. This keeps the old system running while your engineers, supported by AI tools, confirm its business rules carry over to the new one.
Key Takeaways
- The highest cost is opportunity cost, not maintenance
- Five approaches, from smallest to biggest change: rehost, replatform, refactor, rearchitect, rebuild
- AI makes rebuilding realistic for many mid-sized businesses
- Change the application first, the database later
- Move module by module so the business keeps running
- Engineers review everything AI produces
What is Legacy Software?
Legacy software is an application that still plays an important part in your business but runs on outdated technology that is hard to change, connect to other systems and support. Software becomes “legacy” when it starts holding the business back. The most common types of legacy software include:
- Monolithic applications: One big, tightly connected codebase.
- On-premises software: Runs on servers you own and look after.
- Outdated languages and frameworks: Technology that is hard to hire for, such as classic ASP, VB6, older .NET Framework or Java versions, unsupported PHP versions, or COBOL.
- Legacy business platforms: Older ERP, CRM or industry software.
- Heavily customised software: Years of patches, understood by only one or two people.
- Spreadsheet-based systems: Business logic kept in Excel models.
Why Does Legacy Software Modernisation Matter Now?
Customer expectations have changed a lot over the last 10 years, and software built 5 to 10 years ago often struggles to keep pace with modern demands. The highest cost of keeping old software isn’t maintenance. It’s the opportunities your business misses.
Legacy software costs you in two ways:
- Maintenance, the cost you can see: industry estimates commonly put the share of IT budgets spent keeping existing systems running at 60 to 80%.
- Opportunity cost, the cost you can’t see: it sits below the surface and rarely shows up in a budget report.
The diagram below shows both.
Maintenance is only the visible part of what legacy software costs
- Industry estimates: 60 to 80% of IT budgets keep existing systems running
- Fewer developers know the technology, and they charge more
- Security patches may no longer be released
- Hosting and licensing costs for old infrastructure
- Staff time lost to manual workarounds
- Customers email for quotes while rivals offer self-service
- No APIs to connect new partners, marketplaces or payment providers
- Staff re-enter the same data into two or three systems every day
- Your data is locked in an old database, so AI can't use it
- Simple changes take weeks, so good ideas sit in the backlog
- No fast mobile experience for customers who expect one
B2B buyers expect the same convenient experience they get when shopping as consumers, including self-service options, mobile access, real-time order updates and quick answers to their questions. If your software can’t meet these expectations, customers may not complain; they may simply switch to a supplier that offers a better experience.
A useful question for your leadership team is: “What could we achieve in the next 12 months if our existing software did not hold us back?” The answer shows the business opportunities you may be missing because of your current software. That gap is your opportunity cost.
What Are The Signs Your Software Needs Modernising?
If making small changes to your software takes too long, your team has to enter the same information into different systems manually, or you cannot offer customers what they need, your existing software may be holding your business back.
Each warning sign in your software has a business cost
Your technology is out of date, and developers are hard to find
Rising costs and growing security risk
Screens are slow, or the system goes down often
Lost productivity every day
Data moves by manual exports and imports because there are no APIs
Wasted time and costly errors
Old protocols and unpatched software
Higher breach risk and Privacy Act exposure
Customers can't get mobile access or self-service
Customers move to suppliers who offer it
Simple updates take weeks
Missed opportunities while competitors move faster
What Are The Five Legacy Modernisation Approaches?
The five common approaches, from smallest to biggest change, are rehost, replatform, refactor, rearchitect and rebuild. The table below compares them.
Move the system as it is to the cloud
Best when: you need a quick infrastructure win and the code is in reasonable shape
Risk: Low
Cost: Low
Move with small changes, such as a managed cloud database
Best when: you want some cloud benefits on a limited budget
Risk: Low to medium
Cost: Low to medium
Restructure the code without changing what it does
Best when: the core logic is sound, but the code is hard to maintain
Risk: Medium
Cost: Medium
Change the design, for example, splitting a monolith into services
Best when: you have scalability or integration limits
Risk: Medium to high
Cost: Medium to high
Rewrite on a modern stack, keeping the business rules
Best when: The technology is outdated, and the user experience is behind
Risk: Medium (lower when staged)
Cost: Medium to high
Two important points to understand:
- Big bang vs incremental modernisation: these are two ways to update software, not types of modernisation. You can update everything at once or one part at a time. Updating one part at a time is usually safer because the existing system keeps running while you test each new part.
- Rehosting and replatforming: these are quick ways to move software to a new environment with few changes. However, the software may still have the same problems and look and work the same as before. Costs may shift to other areas instead of decreasing.
How Has AI Changed Legacy Software Modernisation?
AI can read, explain and convert old code in a fraction of the time it used to take. This greatly reduces the most expensive part of rebuilding: working out what the existing software already does.
In the past, the biggest challenge in updating old software was understanding how it worked. Documentation was often missing, the original developers were no longer available, and important business rules were hidden in the code. Understanding the existing system could take several months before any changes could begin.
AI tools now help experienced engineers to:
- Read large codebases and explain what each module does in plain English.
- Pull out business rules and edge cases, including those hidden in stored procedures and database triggers.
- Map how modules, database tables and integrations depend on each other.
- Write tests that capture how the current software behaves, so you can prove the new version gives the same results.
- Produce a first draft of converted code in a modern language or framework
- Document the existing database structure.
A well-known example is that in 2024 Amazon reported that AI-assisted tools reduced Java upgrades from around 50 developer days per application to just a few hours. This has helped them save an estimated 4,500 developer years of work.
However, AI has its limits. It cannot decide what your business needs, design the best user experience or guarantee that the code is correct. Developers must review its work to ensure it meets business requirements and works as expected.
At Designpluz, engineers review, test, and approve every line of code AI suggests before using it. Client confidential data stays secure and is never sent to public AI services.
Why is Rebuilding Often The Best Option?
When your software is outdated and difficult to use, simply moving it to a new server won’t solve the underlying problems. You will still have old software running in a new location.
Customers and employees now expect software that is easy to use, works well on mobile devices and lets them complete tasks without always needing help. Older systems can also struggle to connect with business tools such as CRM and ERP software, external partners and AI tools. Without reliable data and systems that work together, adding useful AI features becomes difficult.
Outdated technology can also make it harder to find developers to maintain it and to address security risks. Modernising the software solves these problems at their source. With AI speeding up the work of understanding and updating old code, rebuilding has become a more practical option for many businesses.
A rebuild replaces old software, not just its location
Rehost or replatform
- Same old code
- Same old user experience
- Just in a new location
- Costs often move rather than shrink
Rebuild: what you gain
- Modern, mobile, self-service experience
- Cloud hosting that scales with demand
- Easy links to CRM, ERP, partners, AI
- Easier hiring and support
- Security built in from the start
When a rebuild isn’t the right choice
Rehost or replatform
Software works well and only needs better hosting
Replace with SaaS
Standard process and a SaaS product fits well
Rehost now, rebuild later
Budget or timing is very tight: rebuild in stages later
How Do You Modernise Legacy Software with AI, Step By Step?
Phase 1: Understand and plan
Assess your software
Check technology, security and costs.
Ask staff and customers what frustrates them.
Flags code risks
Set your business goals
Decide what success looks like for the business,
such as faster quotes or customer self-service.
Map your software
Document what the old system does.
Build a checklist the new one must match.
Maps rules, tests
Choose your approach
Rehost, rebuild or replace each part
of the software separately.
Phase 2: Build and launch
Modernise the app first
Keep the existing database for now.
Change one major thing at a time.
Move module by module
Start with a low-risk module that matters,
go live, then repeat.
Drafts first code
Plan your data migration
Clean data early and use scripts.
Rehearse, and check record counts.
Test and switch over
Run old and new side by side,
then switch and keep monitoring.
Our key lesson
Keep the database first.
Split it later, once the new app is stable.
Strangler fig
The new system takes over from the old one, module by module.
Data migration: good practice
Profile and clean it, not in the final week.
Use repeatable scripts, not manual steps.
Practise the migration before go-live.
Compare record counts and totals, old against new.
Migrate module by module, where possible.
How Much Does Legacy Software Modernisation Cost?
Costs depend on the size of the software, the approach and the state of your data. Smaller custom rebuilds start from around $40,000 (excl. GST). Enterprise projects are priced on scope.
The main cost drivers are:
- Size and complexity of the existing software
- Number of integrations with other systems
- Volume and quality of data to migrate
- Security and compliance requirements
- How much the user experience is being redesigned
Smaller modernisation projects typically take 4 to 8 months, while large enterprise projects can take more than 12 months and continue to evolve after launch. It is wise to set aside 15 to 20% of the budget as contingency. Delivering the project in stages also spreads the cost over time and lets the business see benefits before making the full investment.
What Are The Most Common Modernisation Mistakes?
Treating it as an IT-only project
Switching everything over at once
Changing the app and database together
Expecting lift and shift to transform
Copying old screens one for one
Trusting AI code without review
Leaving data migration to the end
No support plan after go-live
How Do You Choose a Modernisation Partner?
Look for a partner who:
Has delivered legacy rebuilds and shows real examples
Explains trade-offs instead of pushing one answer
Is clear about AI use and data protection
Delivers in stages with parity testing
Gives you full ownership of code and IP
Supports the platform after go-live
How Designpluz Can Help
Designpluz is a Sydney-based software development company providing legacy modernisation services for Australian businesses. We start with a software audit, replace components gradually so core operations keep running, and continue to support the platform after launch.
Planning a modernisation project? Talk to our team or explore our legacy software modernisation services.
