What is a legacy system?

A legacy system is an application that has grown over many years, still controls important business processes but is no longer technically sustainable. Typical signs are outdated frameworks, hard-to-maintain code and missing interfaces. So this legacy software still works, but it causes rising costs and slows down new requirements.

How much does it cost to replace a legacy system?

The cost of a legacy replacement depends on the size, condition and criticality of the system, which is why we start with an analysis phase with predictable costs. Afterwards, you know the target architecture, effort and sequence and decide on that basis. Maintenance, licenses and specialist knowledge for the old platform disappear once it is switched off, which lowers operating costs permanently.

Can COBOL and mainframe systems be replaced too?

Yes, and it is the most common case in production-critical environments. The difficulty rarely lies in the language, but in the fact that the business logic has migrated over decades into programs, jobs and database procedures and nobody has the overview any more. The same applies to AS/400 applications and monolithic .NET or Java systems, which are technically younger but structurally just as tangled.

How long does it take to replace a legacy system?

The duration depends on system size and data volume: smaller applications are replaced within a few months, production-critical legacy systems need several years. The number of connected interfaces shifts the timeframe more than the amount of code. Because old and new run in parallel, you benefit from the first migrated modules long before the project ends.

What happens to the data from the legacy system during the replacement?

The existing data moves along, and it is verified: we reconcile data between the old and new system before a module takes over responsibility. Historical data is the more laborious part, because it has grown inconsistently over the years. That is why clean-up starts early and not just before the switch-over.

Which technologies and programming languages does MaibornWolff work with for legacy replacement?

We are not tied to any source technology: in replacement projects we encounter COBOL, Delphi, older Java and .NET applications, C/C++, PHP or Python. What matters is not the old language, but the target architecture. We migrate to the technology that fits the future architecture, often to container-based environments with Docker and Kubernetes, to PostgreSQL or to front ends with React and Angular.

Can newer software become a legacy system too?

Yes. Legacy is not a question of age or programming language, but of condition: hard-to-maintain code, outdated dependencies, missing knowledge in the team or an architecture that no longer fits the requirements. Even in a language that is modern today, legacy code builds up again over the years. A replacement is therefore never just a change of language.

White computer keyboard on a blue background; a bright pink cloud of digital circuits rises from the keyboard, connected by fine conductors/wire lines.

Replace legacy system

A legacy system does not retire because it is old, but when it costs your business more than it delivers.

Replacing a legacy system without stopping production

Software that has grown over many years often controls exactly the processes your revenue depends on. That makes replacing it risky: whoever replaces the backbone risks downtime. So it stays. After all, it works, the argument goes. What gets overlooked is how high the maintenance effort has become and how much speed every new requirement costs.

We replace legacy systems while they keep running. First, we reconstruct what your system does in business terms. Then we replace it building block by building block. It is only switched off once the replacement has been proven to do the same job.

From more than 35 years of working with grown systems, we know: the proof is the hard part, not the new build.

Less Technology. Better Business.

Schedule a free consultation

Understanding before change

Before a single line is replaced, we reconstruct the business logic of your legacy system from the code. Even when documentation is missing.

No downtime in operations

Your legacy system keeps running while the new system takes over module by module. Every result is continuously mirrored against the old one.

Operating costs that drop noticeably

Maintenance and licenses for the old platform disappear once it is switched off. New requirements can then be implemented much faster.

Vendor-neutral, no lock-in

We recommend the architecture that fits your case, not the one that fits our utilization. After the analysis, you decide freely.

Your starting point	The path we recommend
The business logic still holds up, but the platform and frameworks are outdated.	Modernize the legacy system instead of replacing it: We renew code and runtime environment as part of a software modernization; the system stays.
The architecture no longer allows new business requirements.	Replace step by step: A new system takes over module by module, and the old one is switched off at the end.
Hardly anyone understands the system any more, and automated tests are missing.	Get clarity first: The Software Health Check assesses code, architecture and risk before you commit.
The structures are so tangled that even major refactoring can no longer fix them.	Replace in one go: Without a basis for further development, a new build is the more economical route.
There are no more security updates for the system, or new legal requirements cannot be mapped in it.	Replace, and with priority: Neither can be fixed permanently in the legacy system.

How we recover and safeguard the business logic

Today, this loss can be repaired. With Agentic Domain Discovery, we reconstruct the business logic from the source code with AI support and make it verifiable. This is not a promise: the more scattered the logic is in the code, the more remains brainwork. We classify what we find together with your business departments.

Your source code does not go to third-party servers. We run the analysis models in an isolated Trusted Zone, without training on your data.

In parallel, the safety net is created. We record today's system behavior as a reference state and automatically compare every change against it. That way you notice immediately when a rule gets lost, not only at the month-end close.

How a replacement works during ongoing operations

We most often replace legacy systems using the strangler pattern: new functions are built alongside the legacy system and go live one by one until nothing accesses the old one any more. No cut-over date on which everything has to go well at once.

What the new system looks like inside depends on the business domain, not the textbook. Whether clearly separated building blocks as in a hexagonal architecture or a lean new build as a custom software solution: this is decided in the target architecture, with a view to what your team can operate itself later on. For critical systems, an on-call team accompanies the conversion.

Inventory and target architecture

We read the code base, data flows and interfaces and walk through them using a concrete transaction, from the input screen down to the database. Whether COBOL on the mainframe, a grown AS/400 application or a monolithic .NET system from the early 2000s: architecture diagrams are often missing, and documentation is rarely up to date. The result is a cut plan: which business area moves when.

First module in parallel operation

Take over module by module

Switch off the legacy system

Question for every provider	Our answer
How do they reconstruct the business logic when documentation is missing?	AI-supported code analysis and reference states instead of questions to people who are no longer with the company.
Have they already replaced production-critical systems, not just peripheral applications?	More than 800 large-scale systems and more than 10,000 person-years in high-end software engineering.
How do they prove that the new system does the same job?	Automated tests against a recorded reference state, checked in every phase.
Do they tie you to themselves or to a platform?	Vendor-neutral, with knowledge transfer in a mixed team. After the analysis, you decide who implements the plan.

Frequently asked questions about replacing legacy systems

What is a legacy system?

A legacy system is an application that has grown over many years, still controls important business processes but is no longer technically sustainable. Typical signs are outdated frameworks, hard-to-maintain code and missing interfaces. So this legacy software still works, but it causes rising costs and slows down new requirements.
How much does it cost to replace a legacy system?

The cost of a legacy replacement depends on the size, condition and criticality of the system, which is why we start with an analysis phase with predictable costs. Afterwards, you know the target architecture, effort and sequence and decide on that basis. Maintenance, licenses and specialist knowledge for the old platform disappear once it is switched off, which lowers operating costs permanently.
Can COBOL and mainframe systems be replaced too?

Yes, and it is the most common case in production-critical environments. The difficulty rarely lies in the language, but in the fact that the business logic has migrated over decades into programs, jobs and database procedures and nobody has the overview any more. The same applies to AS/400 applications and monolithic .NET or Java systems, which are technically younger but structurally just as tangled.
How long does it take to replace a legacy system?

The duration depends on system size and data volume: smaller applications are replaced within a few months, production-critical legacy systems need several years. The number of connected interfaces shifts the timeframe more than the amount of code. Because old and new run in parallel, you benefit from the first migrated modules long before the project ends.
What happens to the data from the legacy system during the replacement?

The existing data moves along, and it is verified: we reconcile data between the old and new system before a module takes over responsibility. Historical data is the more laborious part, because it has grown inconsistently over the years. That is why clean-up starts early and not just before the switch-over.
Which technologies and programming languages does MaibornWolff work with for legacy replacement?

We are not tied to any source technology: in replacement projects we encounter COBOL, Delphi, older Java and .NET applications, C/C++, PHP or Python. What matters is not the old language, but the target architecture. We migrate to the technology that fits the future architecture, often to container-based environments with Docker and Kubernetes, to PostgreSQL or to front ends with React and Angular.
Can newer software become a legacy system too?

Yes. Legacy is not a question of age or programming language, but of condition: hard-to-maintain code, outdated dependencies, missing knowledge in the team or an architecture that no longer fits the requirements. Even in a language that is modern today, legacy code builds up again over the years. A replacement is therefore never just a change of language.

Why MaibornWolff?

As one of the most innovative IT service providers with a great passion for AI, we focus entirely on the project business and individual software development - without our own products. To stay at the forefront, we continuously invest in our team of digital technology engineers and develop digital solutions that are well thought-out, efficient and reduced to the essentials.

Our principle: simplicity instead of complexity. We only develop what is really needed - tailor-made, useful and reliable. Our results speak for themselves. With over 800 large-scale systems and more than 10,000 person-years of experience in high-end software engineering, we are one of the few who can reliably implement even the largest and most complex IT landscapes. Thanks to close partnerships with leading hyperscalers, our customers operate their solutions in today's most modern and powerful environments.

Less technology. Better Business.

See all Services