Migrating off legacy systems
You move off a legacy system without a big-bang cutover, by running the new path beside the old one.
A migration is a change of platform without a change of business. The goal is the same data and the same reports, on infrastructure that is cheaper, faster, and easier to build on. The risk is doing it all at once.
So we migrate incrementally. The new pipeline runs beside the old one, producing the same outputs, and we compare them until they match. Traffic moves over piece by piece, and the old system is retired only when nothing depends on it.
Along the way we re-model what was tangled, tune storage and compute for cost, and document what was undocumented. You end with a platform you own and understand, not a copy of the old mess on new hardware.