The verdict in three sentences
Modernizing a legacy app (VB6, Access, Delphi, WinDev) to the web costs 50,000 to 150,000 EUR in 2026, depending on screen count and accumulated technical debt. The right strategy is the strangler fig: migrate progressively, module by module, rather than a risky big bang. ROI comes from ending dedicated Windows workstations, mobile access and fewer outages — typically a payback within 24 to 36 months.
What drives the cost of a rewrite
Cost depends not on the app's age but on its size and debt. Here is a 2026 order-of-magnitude grid for a rewrite in Amsterdam.
| Factor | Low | Medium | High |
|---|---|---|---|
| Number of screens | < 15 | 15-40 | > 40 |
| Business complexity | Standard | Moderate | Regulated |
| Technical debt | Documented code | Partial | No docs |
| Data migration | Clean | Some anomalies | Corrupted base |
| Estimated budget | 50,000-70,000 EUR | 70,000-110,000 EUR | 110,000-150,000+ EUR |
| Timeline | 4-6 months | 6-9 months | 9-12 months |
Technical debt is the hidden multiplier: an undocumented Access base, with business rules buried in macros, can add 30 to 50 % to the budget in reverse engineering.
Strangler fig: migrate without a big bang
The strangler fig pattern wraps the old system and replaces its functions one by one, until the legacy is "strangled" and removed. Comparing the two approaches:
| Criterion | Big bang | Strangler fig |
|---|---|---|
| Failure risk | High | Low |
| Service continuity | Possible outage | No outage |
| First deliverable | 9-12 months | 6-10 weeks |
| User feedback | Late | Continuous |
| Total cost | Concentrated | Spread, better steered |
| Reversibility | Hard | Module by module |
Strangler fig rarely costs more overall and drastically reduces project risk. Users switch gradually, easing change management.
The concrete ROI of modernization
| Gain | Before (legacy) | After (web) |
|---|---|---|
| Per-user workstation | Dedicated Windows PC | Browser, any device |
| Remote access | Complex VPN | Secure URL |
| Blocking bug/outage | 2-4/month | Rare |
| Maintenance cost/yr | 12,000-20,000 EUR | 9,000-16,000 EUR |
| Field mobility | Impossible | Native (responsive/PWA) |
Ending dedicated Windows workstations and aging licenses often frees 8,000 to 15,000 EUR/yr, not counting time saved by mobile access for field teams.
Mini case study
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Sylvie, IT lead at a 60-person logistics SME in Amsterdam, runs a 2009 Access app: 28 screens, no documentation, 3 outages/month. She quotes the web rewrite at 95,000 EUR as a strangler fig over 8 months. On the gains side: 4 dedicated Windows workstations removed (6,000 EUR/yr), maintenance cut by 5,000 EUR/yr, and 15 hours/week saved by forklift operators via tablet access (i.e. 20,000 EUR/yr at 26 EUR/loaded hour). Total: 31,000 EUR/yr of gains, ROI reached in 3 years, then net benefit every year after.
FAQ
How long does a legacy-to-web rewrite take?
As a 2026 order of magnitude, expect 4 to 6 months for a small app, 6 to 12 months for a mid-size business app. Strangler fig ships a first usable module in 6 to 10 weeks.
Can I keep the old app during migration?
Yes, that is the whole point of the strangler fig: both coexist, users switch module by module, with no service outage.
Is data migration risky?
It is the most sensitive item. A clean base migrates in days; a corrupted or duplicated base can require weeks of cleanup. A prior audit is essential.
Should I rewrite everything or can I convert the code?
Automatic conversion tools (VB6 to .NET, for example) produce hard-to-maintain code. In 2026, a targeted rewrite of business rules gives a far better long-term result.
What is the main risk to avoid?
The big bang: switching everything at once risks a costly failure. Prefer progressive migration with short milestones and continuous user feedback.
Let's scope your project. Send us your screen count, source technology and data state: we cost the rewrite and the strangler fig plan. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
