What makes ECC migrations hard
The technical conversion is the easy part. The cost sits in custom code, data, interfaces and testing, and in deciding what to keep.
The deadline is closer than the plan. Mainstream maintenance ends at the end of 2027, and the extended options cost more and come with conditions. Programmes that start late lose the choice of approach.
Greenfield or brownfield, argued for months. Finance wants history kept, operations wants processes redesigned, IT wants the least risk. Without facts from your own system, the decision becomes a debate.
Custom code nobody wants to touch. Years of Z programs, enhancements and modifications, a large share never executed, and the rest written against tables and transactions that S/4HANA has simplified.
Simplification items found in testing. Business partner conversion for customers and vendors, the new general ledger and material ledger, changed data models. Each one is manageable early and painful late.
Downtime the business cannot accept. A large database and a weekend cutover window that may not be enough. Nobody has measured the real runtime because nobody has run a full rehearsal.
Interfaces assumed to just work. IDocs, RFCs and file interfaces to banks, EDI partners and warehouses need checking against the new release, and some depend on middleware that is also nearing end of maintenance.