Ageing software does not fail overnight. It slows down, becomes hard to evolve, stops receiving security updates, and ends up depending on the one person who still knows how it works. Migration then becomes inevitable — the real question is how to do it without stopping operations or blowing the budget.
Signs your legacy software needs migrating
- The language or framework is no longer maintained (PHP 5, old Symfony versions, Access, Windev…)
- Every change takes weeks and breaks something else
- Data is exported to Excel to be used
- No documentation, and only one developer knows the code
- The software cannot connect to your website, CRM or AI tools
Two or three of these signs justify an audit. Waiting raises the cost: the more technical debt builds up, the longer the migration.
Step 1: audit before deciding
A successful migration starts with a two-to-three-week assessment: map the features actually used, code quality, data structure and volume, dependencies on other tools. You almost always find features nobody uses — no need to migrate them.
Step 2: choose the right migration strategy
Modernise what exists
If the architecture is sound, upgrade, add tests and replace obsolete pieces one by one. It is the lowest-risk path, for example for a Symfony application stuck on an old version — takeover work our Symfony agency does regularly.
Rewrite progressively
If the code is too fragile, rebuild module by module alongside the old software, sharing the same database or syncing both. Each migrated module replaces the old one, with no big-bang switch.
Replace with another tool
If your needs have become standard, a market tool may be enough. Our guide build or buy software helps decide. For a central management tool, the rewrite often means a custom ERP — see custom vs off-the-shelf ERP.
Step 3: migrate data without stopping operations
Data is the heart of the migration. We clean, de-duplicate and transform it into a new model, then run both systems in parallel long enough to validate the results. We describe this approach in migrating from Excel without stopping operations.
A migration is judged by one simple test: your teams kept working throughout.
What does migrating legacy software cost?
Cost depends less on the software’s age than on four factors:
- The functional scope actually used — hence the audit
- Data quality and volume to migrate
- The number of integrations with other tools (accounting, website, CRM, partners)
- The chosen strategy: modernising usually costs less than rewriting, which costs less than a poorly prepared replacement
Also cost doing nothing: lost time, security risk, dependency on one person, missed opportunities for lack of integration. Phase-by-phase estimates, delivered every two weeks, keep the budget under control and let you stop or redirect at any time.
Mistakes to avoid
- Rewriting everything at once, with a single switch-over
- Rebuilding features nobody uses
- Migrating data at the end instead of from the start
- Forgetting to train users
Legacy software migration: FAQ
How long does it take?
A modernisation can take weeks; a progressive rewrite runs over months, module by module, without stopping operations.
Do we have to rewrite everything?
Rarely. The audit usually shows what can be modernised and what is no longer used.
Where to start
Start with an inventory: what the software does, what it costs and what it prevents. For an outside view, take the 2-minute audit or talk to an expert: we will propose the lowest-risk migration strategy for your case.