Legacy migration: how to modernise an ageing IS without breaking everything

Your information system is 10, 15 or 20 years old? Here's a proven method to modernise it progressively without interrupting your business. The legacy syndrome: A legacy system isn't necessarily old, it's a system that has become difficult to maintain, evolve and secure. Symptoms: obsolete technologies, non-existent documentation, a single developer who 'knows how it works', deployment in days. The paradox: the longer you wait, the more expensive migration becomes. Modernisation strategies: Rehost (lift & shift): move to cloud without modifying code. Replatform: partially adapt for cloud (containerisation). Refactor: rewrite critical parts. Rebuild: reconstruct from scratch. Replace: substitute with SaaS or standard ERP. Choice depends on business value, technical complexity and budget. The Strangler Fig method: Inspired by the strangler fig in botany, this method builds the new system around the old one, migrating features one by one. The old system continues operating during transition. Advantages: zero service interruption, progressive validation, rollback possible at any time. Pitfalls to avoid: Big bang (redo everything at once): 70% failure rate. Identical replication: modernising technology without rethinking processes. No tests: migrating without automated safety net. Scope creep. Neglecting historical data migration. 5-step action plan: 1) Complete IS mapping. 2) Component evaluation (business value vs technical debt). 3) Target definition and per-component strategy. 4) Progressive migration with Strangler Fig. 5) Old system decommissioning. Typical duration: 6-18 months.