What is a legacy system? Meaning, signs, and risks.

Your system has been running for years. It works, more or less. But every adjustment takes more time than the last, new integrations no longer succeed smoothly, and no one knows exactly how everything fits together anymore. These are classic signs of recognizing a legacy system within your own organization.

A legacy system is not simply an old system. Legacy only becomes a problem when it holds your organization back instead of supporting it. In this article, you will read how to recognize a legacy system, what the risks are, and what you can do.

What is a legacy system?

A system becomes a legacy system when it becomes difficult to modify, expand, or replace, while the system still plays an important role within the organization.

In many organizations, legacy systems are systems that have been progressively expanded over the years. What once started as a system for a single process is later linked to other systems, reports, portals, or data sources. As a result, the system becomes part of a larger web of applications and processes. The system itself may not change much, but the dependencies around it increase.

An order system is linked to your CRM, billing system, customer portal, and reports. As long as everything works, there is little to worry about. But when you want to make a change to the order system, it has consequences for multiple other systems. A change becomes complex and risky.

The same can be seen in systems for planning, logistics, or customer data. These systems have been expanded over the years with functionality and integrations. The system itself still works, but the environment around it has become complex.

We often see that organizations only realize a system has become legacy when they want to change something. A new feature, an integration with a modern platform, or a migration to the cloud. At that moment, it becomes clear how many systems and processes depend on that single system.

How systems become legacy

Legacy is not created by a single wrong choice. It arises from years of making logical choices that work in the short term, but build up pressure in the long term.

Phase 1: The system is built

A system is built for the situation of that moment. The organization is smaller, processes are straightforward, and the technical history is low. The system works well.

Phase 2: The organization changes

New teams emerge, new processes are introduced, and other requirements come from legislation or customers. Instead of redesigning, organizations build on what already exists. That is understandable. The existing system works, operations must continue, and there is rarely time to start from scratch.

Phase 3: Every choice is aimed at continuity

An extra integration to enable a new process. An adjustment to quickly resolve a customer request. A workaround because a structural solution takes too much time. Individually, these are defensible choices. Together, they make a system increasingly difficult to understand.

Phase 4: Knowledge disappears

Organizations change faster than systems. Teams shift, suppliers change, decision-makers leave. Knowledge disappears, documentation falls behind, and past choices can no longer be easily traced. What was once built with intent is now something that just exists.

We recently saw this with a client: a planning system that was built ten years ago by an external agency. The system still worked fine, but the original developers were long gone. Documentation was outdated. Nobody dared to make major adjustments anymore, because nobody knew exactly what consequences that would have for other systems.

This is how you end up in a situation where a system is indispensable, but no one really knows how future-proof it actually is.

Is your system already legacy? Do the quick check

Do you recognize three or more of the signals below? Then there is a high chance you are working with a legacy system.

1. Adjustments take months instead of weeks
What used to be quick now takes much more time. Testing, coordinating, and assessing risks consumes capacity.

2. Multiple systems depend on this system
A change has consequences for multiple other systems. Nobody dares to adjust anything anymore.

3. No one knows exactly how the system works anymore
The original team is gone. Documentation is outdated or missing. Knowledge resides with a single person, or nowhere at all.

4. You prefer to build new functionality next to it
You deliberately build new features in a separate system because adding them to the old system is too complex.

5. Maintenance costs more than continuous development
The majority of your IT budget goes toward keeping what exists up and running. Little is left for innovation.

6. Integrations with modern tools do not work
You want to connect to a modern CRM, a cloud platform, or a new API, but the old system does not support this without custom work.

7. Releases take months
What used to take weeks now requires a whole process of preparation, testing, and alignment.

Why don't organizations replace legacy systems?

If everyone knows that legacy systems bring risks, why don't organizations just replace them? In practice, there are good reasons why this doesn't happen.

The system still works

Orders are processed, invoices are sent, reports are generated. As long as the organization can keep running, replacement rarely feels like the highest priority. Problems are solved with small adjustments. The system persists.

New functionality always takes priority

The business wants new functionality, new products, and improvements for customers. This delivers immediately visible value. Replacing an existing, internal system delivers few visible benefits in the short term, while it costs a lot of time and money. New projects take priority.

Replacement is risky

Legacy systems are often at the core of your organization. If something goes wrong during a migration, it can have immediate consequences for customers, orders, invoicing, or reports. The risk of replacement feels greater than the risk of continuing to work with the existing system.

Example from our practice: TNO had an intranet that no longer aligned with how the organization worked. Information was scattered across 3000 pages, employees no longer knew where to find anything, and the system stood in the way of further development. Replacement was necessary, but the challenge was not in the technology. The biggest question was: which solution has the least impact on the organization?

We guided the project in the role of Senior Project Manager. We conducted an impact analysis on the choice of platform, coordinated the content migration, and developed a governance framework to ensure the new intranet remained usable. The project took 19 months. The result: a platform that supports internal collaboration and communication instead of hindering it.

It is not an IT project but an organizational change

Replacing a legacy system means that processes change, data is cleaned up, reports are redesigned, and teams work differently. It affects multiple departments. It quickly becomes an organizational project rather than just a technical one.

No one really owns the entire system

In many organizations, a legacy system has been built and modified by multiple teams over the years. The business owns the process, IT owns the technology, and management decides on budgets. As a result, there is often no single person who is fully responsible for replacing the entire system, leading to postponed decisions.

Postponing makes the problem bigger

Because replacement is complex, risky, and expensive, many organizations choose to adjust systems step-by-step instead of replacing them completely. In the short term, that is logical. In the long term, this makes systems even more complex and legacy grows further.

That is why many legacy systems remain in place longer than organizations would actually want. 

What are the risks of legacy systems?

Legacy systems are not by definition a problem. Many organizations run for years on systems that are technically legacy. The risks only become visible when systems need to change, need to be linked to new technology, or when you want to grow.

Security risks increase

Older systems sometimes run on technology that is no longer actively supported. Security updates become less frequent. Security vulnerabilities arise. It becomes harder to comply with new security guidelines or laws and regulations. Especially when legacy systems contain important company data, this is a major risk.

Integrations with modern systems are difficult

Legacy systems are often not built to work together with modern applications, cloud solutions, or new platforms. You have to build custom solutions or create temporary workarounds. This makes systems more complex and more expensive.

User-friendliness falls behind

Many legacy systems were built at a time when user-friendliness was less of a priority. Employees have to work with complicated screens, use multiple systems, or manually transfer data. This takes time and increases the chance of errors.

Maintenance takes more and more time

Older systems are harder to modify. Small changes take relatively much time because developers have to take existing functionality and old choices into account. IT teams spend more and more time on maintenance and less on innovation.

Changes take longer

When systems are hard to modify, changes take longer. New products, new processes, or new reports take more time to develop. The organization becomes less flexible. It takes longer to respond to changes in the market.

Replacement becomes increasingly complex

The longer a legacy system exists, the more processes, data, and systems depend on it. Replacing it becomes larger and more complex. Many organizations postpone replacement. The problem grows.

What can you do with legacy systems?

When an organization works with legacy systems, there are usually a few options. Here they are:

1. Continue to use and maintain the legacy system

Sometimes the system is stable, supports a clear process, and little changes. Then it can be perfectly fine to leave a system as it is and only maintain it. However, it is important that you know what the risks are and that you continue to monitor and document the system.

2. Modernize the legacy system step-by-step

In many organizations, a system is not replaced all at once, but adjusted step-by-step. For example, by rebuilding parts, moving functionality to new systems, or replacing integrations. This is often done to limit risks and gradually modernize systems.

3. Build a new system alongside the old system

Sometimes organizations choose to build a new system and slowly phase out the old system. New functionality is then only built in the new system, while the old system continues to run for existing processes and data.

4. Replace the system completely

In some situations, replacement is ultimately the best choice, for example, when maintenance becomes too expensive, security risks become too great, or the system holds back the growth of the organization. These are often large projects that can take several years and have a major impact on the organization.

Which approach fits best depends on your situation, your risks, and your budget. Read more about how to tackle legacy step-by-step on our page about legacy and modernization.

Legacy systems and AI: why modernization is becoming more urgent now

Many organizations want to use AI to speed up processes or reduce costs. But those who want to do so with a legacy system as a foundation quickly run into obstacles.

AI needs access to data. Clean, structured, up-to-date data. Legacy systems often store data in outdated formats, scattered across multiple locations, without proper documentation. Connecting to an AI tool or a modern data platform is then technically complex and expensive.

In addition, AI applications require rapid iterations. Quick testing, adjusting, and deploying. That does not fit with a system where every adjustment takes months of preparation.

Organizations that want to start with AI now, therefore often begin with their legacy systems. Do you want to use AI responsibly alongside your existing systems? Then also read why existing IT frameworks fall short for AI governance.

So…

A system becomes legacy when adjusting, expanding, or replacing it becomes difficult and changes in your organization are slowed down.

Many legacy systems were once well-designed and functioned well for years. Legacy arises because organizations change, systems are expanded, and IT landscapes become more complex. Systems become interconnected. Replacing or adjusting them becomes more difficult.

Organizations often only recognize legacy systems when changes take a long time, risks increase, or the system becomes a barrier to new developments. At that point, the system is already deeply interwoven with other systems and processes. Replacing or modernizing becomes a complex process.

The earlier you recognize that a system is starting to become legacy, the more options you have. Modernizing, replacing, or otherwise organizing systems step-by-step becomes easier if you address it early.

Want to know if your systems are becoming legacy or need help with modernization? Get in touch with Maarten. He would love to think along with you.

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