The jump from SAP ERP 6.x to S/4 HANA is technically and functionally quite large. There are a number of changes, which should be considered at the changeover, and affect existing functionality.
Technical Changes (excerpt)
Functional Changes (excerpt)
Module-spanning parameters like e.g. vendor/customer, material number, messages, documents, are going to be modified. For example there will no longer be a vendor/customer, only one central business partner.
The use of the “new ledger” is mandatory
Functions of previous external SAP systems (e.g. EWM, APO, GTS) are partly going to be integrated in the S/4HANA core.
Single transactions will only be available as Fiori app in the future
The aim of all these changes is to provide exactly one solution for one business process, instead of several transactions for the same process. The SAP landscape will be simplified, too. Many add-ons, like EWM, are integrated in the new business suite and are now a component of S/4HANA. This saves money and effort on the infrastructure.