S/4HANA migration: how to plan a transformation that actually lands

Start with the decision, not the tooling

Most S/4HANA programmes lose their first three months debating tools instead of scope. The decision that shapes everything else is the conversion route: brownfield (technical conversion of the existing ECC system), greenfield (a new build on clean master data and standard processes) or selective data transition (a new shell with chosen history and objects migrated in).

Brownfield keeps history and custom code and is usually the fastest path to a running S/4HANA system. Greenfield gives the cleanest process baseline but forces every business rule to be re-agreed. Selective transition sits between the two and costs the most to orchestrate.

Run the readiness analysis before you commit

The SAP Readiness Check, custom code analysis and simplification item list give a factual picture of what stands between the current system and S/4HANA. Treat the output as a work breakdown, not a report: every simplification item is a decision with an owner, and every custom object is either adapted, replaced by standard or retired.

• Custom code volume and how much of it is genuinely used
• Simplification items affecting finance, logistics and master data
• Interfaces and their owners, including partner-facing IDoc flows
• Add-ons and third-party components with their S/4HANA compatibility status

Sequence the programme around finance and logistics

The universal journal changes finance first, so finance defines the cutover calendar. Logistics, EWM and TM streams follow the finance milestones rather than running on their own schedule. A realistic mid-size programme runs 12 to 18 months from preparation to go-live, with two full test cycles and at least one dress-rehearsal cutover on production-scale data.

Size the team honestly

Under-sizing the technical stream is the most common planning error. A mid-size conversion needs a Basis and technical lead through the whole programme, ABAP capacity for custom code remediation, functional leads per module, an integration owner and a test manager. Peaks around test cycles and cutover need extra hands for a few weeks — exactly the profile where flexible specialist capacity is cheaper than permanent hires.

Protect the run while you transform

The existing landscape keeps producing invoices, deliveries and payroll while the project runs. Keeping AMS and project work in one delivery agreement avoids the classic gap where the support team blames the project and the project blames support.

What good looks like at go-live

A successful cutover is boring: rehearsed twice, timeboxed, with a documented rollback point and an agreed hypercare window with named people on call. Everything that surprises you at cutover was already visible in a test cycle nobody had time to analyse.

Frequently asked questions

How long does an S/4HANA migration take?

A mid-size brownfield conversion typically runs 12 to 18 months from preparation to go-live, including two full test cycles and a rehearsed cutover. Greenfield builds usually take longer because every process has to be re-agreed.

Brownfield or greenfield — which is cheaper?

Brownfield is normally cheaper and faster because history and working custom code are carried over. Greenfield costs more up front but removes legacy process debt, which can pay back over the life of the system.

Do we need external specialists for the conversion?

Most organisations cover the functional knowledge internally and buy in Basis, ABAP remediation and integration capacity for the peaks around test cycles and cutover.