November 11, 2026
November 11, 2026
Almost every established company has one: the system nobody wants to touch. It still runs the orders, the stock or the accounts, but it's slow to change, hard to connect to anything new, and only one or two people really understand how it works. Replacing it feels risky. Keeping it feels expensive. This guide explains the main ways to modernize a legacy system, what drives the cost, which risks to watch, and how to choose an approach that fits your business.
Seven ways to modernize a legacy system
A legacy system is software or hardware that a business still relies on but that has become difficult or expensive to maintain, change or connect with newer technology. Age is part of it, but not the whole story. A ten-year-old system that's well documented and easy to update isn't really a problem. A five-year-old one built on an abandoned framework, with no tests and no documentation, can already be legacy.
Typical examples include:
Legacy modernization is the process of updating these systems so they're easier to maintain, safer to run and better able to support the business. It doesn't always mean starting again. Sometimes a well-placed API or a move to the cloud solves most of the problem.
Most businesses don't modernize because of a single event. The pressure builds slowly. Here are the signals we see most often:
If several of these sound familiar, modernization is probably already costing you more than you realise.
Modernization projects cost money, so it's tempting to keep the old system running "for one more year". But keeping legacy systems alive has its own price, even if it's spread across many budget lines.
The US Government Accountability Office (GAO) offers a useful reference point. In a 2019 report, it noted that about 80% of the federal government's IT budget, which was over $90 billion that year, went on operating and maintaining existing systems, including ageing legacy ones. That leaves only a small share for anything new. Most businesses won't be at that extreme, but the pattern is familiar: the more you spend keeping old systems running, the less you have for improvement.
The hidden costs of legacy systems usually include:
None of these appear on a single invoice, which is why they're easy to underestimate.
A widely used framework, popularised by the research firm Gartner, describes seven options, often called the "Rs" of modernization. They range from light changes to a full replacement. The right choice depends on what's actually wrong with your system: the infrastructure, the code, the architecture or the fit with your business.
Encapsulate
Encapsulation keeps the existing system in place but wraps it with APIs, so newer applications can read and write its data in a controlled way. For example, a distributor might keep its 15-year-old order system but add an API layer so its new customer portal and mobile app can check stock and place orders directly.
It's often the fastest and cheapest first step, and it reduces pressure on the old system while you plan the next move. The downside is that the old code is still there, with all its limitations. Our guide to what system integration involves explains the different ways systems can be connected.
Rehost
Rehosting, also called "lift and shift", moves the application to new infrastructure, usually the cloud, with little or no change to the code. It solves problems such as ageing servers, an expensive data centre or poor resilience. It doesn't make the application itself any easier to change, but it can be done relatively quickly and creates a stable base for further work. A good cloud hosting setup and careful remote system migration are the key ingredients.
Replatform
Replatforming is a step further. The application moves to a new platform with small, targeted changes, such as switching to a managed cloud database or a newer runtime version. You get some of the benefits of the cloud, such as easier scaling, automatic backups and less server maintenance, without a full rewrite.
Refactor
Refactoring improves the internal structure of the code without changing what the system does for users. Developers remove duplication, break up very large modules, add automated tests and update outdated libraries. It's the right choice when the system does its job well but has become slow and risky to change. Refactoring is often done gradually, alongside normal feature work.
Rearchitect
Rearchitecting changes how the system is built, for example splitting one large application into smaller services that can be developed, deployed and scaled separately. This makes sense when the business needs far more scale or flexibility than the current design allows, such as a retailer whose online orders have grown tenfold. It's a significant project and requires strong engineering skills.
Rebuild
Rebuilding means rewriting the application from scratch with modern technology, while keeping its purpose and core business rules. It's appropriate when the existing code can't reasonably be saved, but the system still does something specific to your business that no off-the-shelf product handles well. A rebuild gives you the most freedom, and it carries the most risk, which is why it's often done in stages rather than all at once. This is typically a custom software development project.
Replace
Replacing means retiring the old system and moving to an existing product, often a SaaS platform. This works well when your processes are fairly standard, such as accounting, HR or basic CRM, and good products already exist. The main work then shifts to data migration, integration and adapting your processes to the new tool. If you're weighing a packaged tool against a custom build, our comparison of low-code and custom software development may help.
In practice, many projects combine several approaches. A company might replace its old accounting module with a SaaS product, rehost its order system in the cloud, and encapsulate it with APIs so a newly built customer portal can talk to it.
There's no standard price, because the work ranges from adding a few APIs to rebuilding a core system. What you can do is understand how the approaches compare and what drives the cost within each one.
The main cost drivers are:
A sensible way to manage cost is to start with an assessment. A few weeks spent mapping the system, its data, its users and its connections usually pays for itself by preventing the wrong approach. Our consulting services often begin exactly there.
Modernization projects fail more often from poor planning than from bad technology. These are the risks that deserve the most attention.
The moment you move from the old system to the new one is the most dangerous point in the project. The UK bank TSB learned this in April 2018, when it migrated customer accounts to a new IT platform. The platform ran into serious problems immediately, and many of its 5.2 million customers were affected across branches, phone, online and mobile banking. It took until December 2018 to return to normal operations. In 2022, UK regulators fined TSB a total of £48.65 million, finding that the bank had failed to organise and control the migration programme adequately and to manage the risks from its outsourcing arrangements.
Few businesses face that scale, but the lesson applies to everyone. Avoid a single "big bang" switch where possible. Migrate in stages, run old and new systems in parallel for a period, rehearse the cutover, and have a tested plan for rolling back if something goes wrong.
Legacy systems often contain years of small rules that nobody wrote down: special pricing for one customer, a tax exception, a workaround for a supplier's file format. If a new system misses them, problems appear weeks after launch. The fix is careful discovery: reading the code, interviewing long-time users and comparing outputs from old and new systems using real data.
Old data is rarely as clean as people expect. Duplicate customers, missing fields and inconsistent formats are normal. Plan data cleansing as a project in its own right, migrate test batches early, and check the results with the people who use the data every day.
Once a modernization project starts, everyone has a wish list. Adding new features during the move makes the project longer, more expensive and harder to test. A practical rule is to first match what the old system does, then improve it in later phases.
A technically perfect system still fails if staff don't use it. Involve users early, train them before launch, and keep a feedback channel open in the first months.
A widely used technique for reducing most of these risks is the "strangler" approach. Instead of replacing the whole system at once, you build new functions around the old system piece by piece, gradually moving users and data across until the old system can be switched off. It takes longer, but each step is smaller and easier to reverse.
Every project is different, but most successful modernizations follow a similar path.
1. Assess what you have. List the systems, their users, their data, their connections and their pain points. Note which parts are critical and which are rarely used.
2. Define what the business needs. Be clear about the goal: lower running costs, better security, faster changes, new integrations or support for growth. The goal shapes the approach.
3. Choose an approach for each part. Not every module needs the same treatment. Some can be retired, some encapsulated, some rebuilt.
4. Start with a pilot. Pick a part that matters but isn't the most critical, and modernize it first. You'll learn a lot about the data, the hidden rules and the team before tackling the core.
5. Migrate in stages. Move users, data and functions gradually, with testing and the option to roll back at each stage. Quality assurance and testing is especially important here, including comparisons between old and new outputs.
6. Decommission the old system. Once everything has moved and been verified, switch off the old system properly. Archive the data you're required to keep and cancel the licences and contracts you no longer need. Many projects skip this step and end up paying for two systems.
Serdao has been building and supporting business software for more than 18 years, working with over 50 technologies. Our model combines European project management from our headquarters in France with development teams in Ho Chi Minh City, Vietnam.
We can help with:
If you're also moving systems to new servers or locations, our article on remote system migration explains how that work is typically planned.
Have a system you're not sure what to do with? Explore our software development services or contact our team to talk it through.
Conclusion
Legacy modernization doesn't have to mean a risky, all-or-nothing rewrite. For many businesses, the best path starts small: an API layer to connect the old system to new tools, a move to the cloud, or the replacement of one module that's clearly holding everything back.
What matters most is understanding what you have before deciding what to change, choosing an approach for each part of the system, and moving in steps you can test and reverse. That way the business keeps running while its systems catch up.
1. What is legacy system modernization?
It's the process of updating outdated software or infrastructure so it's easier to maintain, more secure and better able to support the business. It can range from adding APIs to an old system to moving it to the cloud, rebuilding it or replacing it with a new product.
2. What are the main approaches to legacy modernization?
The seven commonly cited options are encapsulate, rehost, replatform, refactor, rearchitect, rebuild and replace. Many projects combine several of them across different parts of the system.
3. How long does a legacy modernization project take?
It depends on the approach and the size of the system. Adding APIs or rehosting can take weeks to a few months. Rebuilding or replacing a core business system usually takes many months and is often split into phases.
4. Is it better to rebuild or replace a legacy system?
Replace it if your processes are fairly standard and good products already exist. Rebuild it if the system supports a way of working that's specific to your business and that off-the-shelf products can't handle well.
5. What is the biggest risk in legacy modernization?
Disruption to the business during the switch from old to new, often caused by data problems or business rules that were missed. Migrating in stages, running systems in parallel and having a rollback plan reduce this risk significantly.
6. Can we modernize without stopping operations?
Yes, in most cases. Incremental approaches move functions and users across gradually, so the old system keeps working until each part of the new one has been tested and accepted.
7. How do I know which approach is right for my system?
Start with an assessment of the system's code, data, integrations and users, and be clear about the business goal. That analysis usually points to the right approach for each part, and it's far cheaper than choosing the wrong one.