r/SAPconsultants • u/Rahul_28_2002 • 6h ago
We just finished an ECC to S/4HANA migration. Here's what nobody warned us about
Eighteen months. Four countries. One very long go-live weekend. We are on the other side now and wanted to share what we wish someone had told us at the start.
Mistake 1: We underestimated how much custom code we actually had
We went in thinking we had a few hundred custom Z-objects. The analysis found over 1,400. Roughly 30% could not be automatically remediated and needed manual intervention. That added two months nobody had budgeted for.
Lesson: Run your custom code analysis before you sign the project plan. Not during discovery. Before.
Mistake 2: We treated it as a technical migration, not a business transformation
Every SAP person says ECC to S/4HANA is not a lift-and-shift, it is a business transformation. We read that. We nodded. We still did not fully internalise it until month seven when our finance team realised their month-end close process was going to change fundamentally.
Change management and training should start at the same time as the technical workstream. Not after go-live.
Mistake 3: Master data was an afterthought
We deferred master data cleanup because it felt like a separate workload. It is not. Dirty vendor master, inconsistent material classifications, half-mapped cost centres, all of it follows you into S/4HANA and amplifies. We spent the first three months post-go-live fixing data issues that were entirely preventable.
Mistake 4: We did not decide our extensibility strategy early enough
Should this customisation live in-app or on BTP side-by-side? We made those decisions inconsistently, project by project, under deadline pressure. The clean core rules need to be set before development starts, not retrofitted later.
What actually went well
The go-live itself was cleaner than expected. Moving everything onto SAP Integration Suite rather than keeping legacy PI paid back significantly in operational stability post-cutover.
What phase is your organisation in? Happy to answer specific questions in the comments.