Replacing Legacy Software: Costs, Risks and a Step-by-Step Roadmap

Four strategies – not every old system needs a rewrite

The most common mistake in legacy projects: a complete new building (rewrite), although a cheaper strategy leads to the same result.

Strategy 1: Encapsulate (API capsule)

Short: The old system remains unchanged.

The old system remains unchanged. An API layer is built to connect modern applications (new web UI, mobile app, external partners). The old system is no longer visible.

• *When: Core logic is stable, but the UI or interfaces are outdated. Costs: 10,000–30,000 € Risk: Low – old system is not touched.

Strategy 2: Rehost

Short: The system relies on new hardware or cloud infrastructure.

The system relies on new hardware or cloud infrastructure. Software remains identical. Profit: lower operating costs, better access, end of dependency on outdated hardware.

Costs: 5,000–25,000 €

Strategy 3: Replatform

The system is upgraded to a new runtime environment.

E.g. from .NET Framework 4.8 to .NET 8, from PHP 5 to PHP 8, from Delphi 6 to Delphi 12. The code is adjusted.

However, the architecture remains preserved.

Costs: 20.000–80.000 €

Strategy 4: Rewrite (targeted new development)

Short: Only where the old system is no longer structurally viable.

Only where the old system is no longer structurally viable. In practice, this is rarely the entire system, but custom modules.

What does an legacy system assessment cost?

Before anyone can call a budget for modernization, the old system must be understood. This is not a precaution – it is a duty.

A professional Altlast review includes:

• Code architecture: All undocumented dependencies and business logic decisions.
• Data assessment: Quality, completeness, data base.
• Technology assessment: What runtimes, libraries, operating systems are affected? Risk assessment: What happens if a specific component fails?
• Recommendation report: What strategy makes economic sense?

Cost Legacy system assessment: 3,000–12.000 € (typically: 5 working days fixed price)

The report contains a decision template with budget frameworks for modernization options – no non-binding films.

Data Migration: the underestimated cost factor

In a Delphi system that was used for 15 years, there is often more business knowledge in the database than in the application itself.

Calculation rules that are not recorded elsewhere. Special treatments for custom customers. Historical data that must be stored for GoBD.

Data migration includes:

• Mapping tables (source → target, incl. transformation rules).
• Staging environment for migration trials.
• Reports (Old stock = New stock?)
• Cutover concept with freeze window and rollback plan.

Data migration typically costs 15–30 % of the total project budget.

Proven migration schedule (parallel operation)

Platform migration (Replatform) 45.000 €

Legacy system assessment 2–4 weeks/span>

Code architecture: undocumented business logic, dependencies, data quality

∅ 45.000 €

Target image & Strategy · 1–2 weeks/span>

What strategy fits? Which modules first? Risk assessment

Partial rebuild (Replatform+) 85.000 €

Data staging & mapping · 2–4 weeks/span>

data migration concept, mapping tables, quality check

∅ 85.000 €

Incremental development (iterativ) · 3–12 months

module for module, parallel operation, permanent adjustment

Full rebuild (Rewrite) 150.000 €

Parallel Operation & Tests 1–3 months <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <– <–

Old system and new system run parallel, results are compared

∅ 150.000 €

Cutover & shutdown · 1–2 weeks/span>

Freeze window, Delta-Sync, Rollback plan, Altsystem shut off

Practice Example: Delphi 6 → .NET 8 (Production Control)

For a manufacturer with 200+ machine connections, we migrated a 15 year old Delphi-6 production control into a .NET-8 web app.

Parallel operation	6 months (no production stop)
Migration duration total	14 months
Project volume	about 95,000 €
Maintenance expenditure after migration	–70 %
Machine connections via OPC-UA	200+

“A Delphi system running for 15 years is not broken – but it is invisible. The first phase is always archaeology." – Björn Groenewold, Dipl.-Inf.

Frequently asked questions

Can we continue working during migration?

Yes – that is even the normal case. Parallel operation (old system and new system running simultaneously) is the gold standard for critical productive systems.

Only when all tests are green, the old system is switched off.

What happens if the old system is only understood by one person?

We call this "knowledge risk' and is an separate part of the Altlast review.

We conduct structured interviews, document implicit knowledge and create a knowledge repository – before the person retires or leaves the company.

How long does a typical legacy project last?

Capsulation/Rehost: 4–10 weeks. Replatform: 3–8 months. Full rewrite: 9–24 months. With parallel operation at the upper limit, without at the lower.

What does it cost to do nothing?

This is a serious question. Unpatched systems are security risks. Outdated runtimes lose support.

If the only developer who knows the system leaves the company, it can be existentially threatening.

We call this the 'Stylish Price' and calculate it explicitly in our recommendations.

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*Groenewold IT Solutions, Empty – Legacy Modernization for the German mid-sized firms since 2012. Request legacy system assessment. *

Technical sources and further links

The following separate references complement the grouping on the topics of this Article:

• Bitkom – Digital Economy Association.
• BSI – Federal Office for Information Security.
• European Commission – Digital Strategy.
• MDN Web Docs (Mozilla)
• W3C – World Wide Web Consortium.

About the author

Björn Groenewold(Dipl.-Inf.)

Managing Director of Groenewold IT Solutions GmbH and Hyperspace GmbH

Since 2009 Björn Groenewold has been developing software solutions for the mid-market. He is Managing Director of Groenewold IT Solutions GmbH (founded 2010) and Hyperspace GmbH. As founder of Groenewold IT Solutions he has successfully supported more than 250 projects – from legacy modernisation to AI integration.

Software ArchitectureAI IntegrationLegacy ModernisationProject Management

More from BjörnSchedule consultationLinkedInXingInstagramProfileProfileProfileAmazonProfile

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Replacing Legacy Software: Costs, Risks and a Step-by-Step Roadmap addresses a practical choice for product and IT teams. Start with one clear goal: reduce legacy risk in controlled steps while daily work and key data remain available.

Check the current process, the data involved, and the result users need. Then record the main risks and define a small first step. This keeps the decision easy to review and gives your team a shared basis.

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