The verdict in three sentences
Rewriting a legacy internal tool (Access, VB6, giant Excel) to a web application costs EUR 60,000 to EUR 180,000 in 2026 over 6 to 12 months, data migration and training included. The real risk is not the rewrite cost but that of staying on legacy: blocking outage, single-person dependency, inability to evolve. The phased approach delivers value every 6 to 8 weeks and smooths the budget.
Scope and cost by phase
| Phase | Content | Cost (2026 estimate) |
|---|---|---|
| Audit & scoping | Reverse engineering, data mapping | EUR 6,000 - 15,000 |
| Architecture & core | Auth, database, design system, CI/CD | EUR 12,000 - 30,000 |
| Business modules (phased) | Progressive functional rewrite | EUR 25,000 - 90,000 |
| Data migration | Cleansing, mapping, load, checks | EUR 8,000 - 25,000 |
| Training & change | Documentation, sessions, support | EUR 5,000 - 15,000 |
| Testing & cutover | QA, parallel run, go-live | EUR 6,000 - 20,000 |
Data migration is the riskiest item: a legacy tool accumulates 10 to 20 years of inconsistencies that must be cleaned before load.
The cost of staying on legacy
| Risk | Frequency / impact | Estimated cost (2026 estimate) |
|---|---|---|
| Blocking outage (activity stops) | 1-3 days/year | EUR 5,000 - 40,000/incident |
| Single-developer dependency | Departure = full lock | EUR 30,000 - 100,000 risk |
| Inability to evolve / integrate | Missed opportunities | Hard to price, high |
| Security & compliance | Breach, GDPR non-compliance | EUR 10,000 - 150,000 |
| Lost agent time (double entry) | 20-40% unproductive time | EUR 15,000 - 60,000/year |
Legacy looks "free" because amortised, but it bills silently: incidents, overstaffing, lost opportunities. The rewrite turns this hidden cost into a controlled investment.
The phased approach over the big bang
| Approach | Big bang | Phased (recommended) |
|---|---|---|
| Delivery | All at the end | Every 6-8 weeks |
| Risk | High (tunnel effect) | Controlled |
| Cash flow | Single large payment | Smoothed budget |
| Adoption | Change shock | Progressive |
| Rollback | Difficult | Possible per module |
The big bang concentrates all risk on a single cutover. The phased approach delivers value early, validates each module in production and reassures users.
Mini case study
Raj, IT director of a mid-sized manufacturer in Singapore, depends on an Access tool maintained by a single ageing vendor. He launches a phased web rewrite at EUR 120,000 over 9 months. Result: double data entry disappears, agent time drops 30% across a 12-person team at ~EUR 38,000 loaded/year, i.e. ~EUR 137,000 of recovered capacity per year. Add the end of blocking outages and single-person dependency: the investment is repaid in the first year.
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FAQ
Can we keep the old software during the rewrite?
Yes, it's even recommended: run legacy and the new app in parallel module by module until full cutover. This secures the business.
How long does a legacy rewrite take?
Between 6 and 12 months depending on size and module count. The phased approach delivers the first modules within 6 to 8 weeks.
What about 15 years of existing data?
You audit, cleanse and migrate via controlled mapping. Budget 10 to 15% for this load, often underestimated.
How do we limit the risk of failure?
Serious scoping, phased approach, parallel run and full transfer of code and documentation. Avoid vendors who keep control of your software.
Is custom mandatory, or will an ERP do?
If your processes are standard, a market ERP may suffice. If they are your competitive edge, custom protects that specificity while modernising the core.
Let's scope your project. Describe your current software, its data volume and critical modules, and we'll frame a phased rewrite. 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.
