Big-bang replacements are the most expensive way to modernise. Routing traffic one path at a time is slower on paper and far cheaper in practice.
Everyone knows the story: three years, a nine-figure budget, and a system that does less than the one it replaced. The alternative is not glamorous, but it works.
Put a seam in front of it
A routing layer between your users and the old system costs weeks, not years. From that point on, every route can move independently.
Move the cheapest risky thing first
Not the biggest, not the easiest — the one that teaches you the most about the seam while still being safe to get wrong.
Run both until the numbers agree
Parallel running is tedious and it is what makes the cutover boring. Reconcile daily, and do not switch anything off until the difference is zero.
Decommission on purpose
An old system nobody has switched off is still costing you licence fees and audit scope. Plan the archive before you plan the migration.
It takes longer to describe than a replacement. It takes considerably less time to survive.
