Every SAP ECC customer knows the dates by now: mainstream maintenance ends in 2027, and even the extended-maintenance option runs out in 2030. What fewer organisations have internalised is what the calendar does to programme quality. Migrations planned in 2026 are assessments, choices and phased programmes. Migrations planned in 2029 are whatever can be contracted in time.
The first decision is not greenfield versus brownfield — it is understanding what you actually run today. A serious readiness assessment covers four things: how much of your custom code is still executed at all; how far your processes have drifted from SAP standard; the state of your master data; and which interfaces and bolt-ons your business quietly depends on.
In our assessments it is common to find that 40–60% of custom code has not been executed in the last year. That single finding changes the migration conversation: you are not carrying your whole history forward, only the part that is alive.
From there, the route decision becomes evidence-based. Heavy process drift and poor master data push towards greenfield — a clean reimplementation. A disciplined landscape with valued customisations favours brownfield conversion. Many real answers are selective: a new core, with surgical carry-over of what has proven value.
Whichever route, the calendar argument is the same: assessment now costs weeks and buys you options. Assessment later costs the options. If your organisation runs ECC and has no written S/4HANA position yet, the assessment — not the migration — is the urgent step.