Long-lived systems accumulate knowledge. Over the years, reports get added, integrations get patched in and business rules end up living in places nobody remembers. When it's time to modernise, that hidden logic is often one of the biggest sources of risk.
Start with strategy and readiness
Before getting into extraction and mapping, it helps to be clear on why you're moving and what good looks like afterwards. What should the new environment make easier? What are you prepared to change about how people work? Who owns the decisions when the old and new systems disagree? Those answers shape everything that follows.
Make the invisible visible
- Dependency inventory: the data, reports, interfaces, scheduled jobs and spreadsheets that rely on the current system, and who relies on them.
- Embedded business rules: logic sitting in code, views, stored procedures and workarounds people have quietly built.
- Retirement decisions: what moves, what gets rebuilt differently and what is switched off, agreed with the people who use it.
- Reconciliation: how you will prove the new system matches the old one, including which totals and records get checked and by whom.
- Cutover and rollback: the order things move, the freeze window, how long both systems run in parallel and what triggers a rollback.
A smaller, safer migration
This groundwork often shrinks the migration. Unused objects get retired rather than rebuilt, and effort goes where it matters. It also gives decision-makers a clearer picture of cost, sequencing and risk before anything gets signed.
None of this is glamorous. But it's usually far cheaper to learn what the old system does before you switch it off than after.