ESB modernisation fails in a predictable way: the programme tries to move everything at once, the cutover weekend runs out of hours, and the rollback leaves the estate running two platforms permanently.
We sequence it instead. Current-state architecture analysis and a to-be design come first, then the interfaces move in slices — grouped by shared dependency rather than by business convenience — with the legacy bus proxying whatever has not moved yet. The old platform is decommissioned progressively, and every slice is independently reversible.
It is slower to describe in a steering committee and considerably faster to finish.
Most of our migration work has been on and around IBM webMethods — Integration Server, Trading Networks, Broker and Universal Messaging, the EDI module, and version moves across the 9.x to 10.x line, including the side-by-side hops those usually need. That is also where the awkward parts live: partner agreements and processing rules that have to be recreated rather than exported, and flow services carrying configuration inline that only shows up after promotion.