The deployment options that allow modification of SAP source code, though not recommended, include SAP S/4HANA (on-premise) and SAP S/4HANA Cloud private edition. Both these options provide the flexibility for organizations to modify SAP source code to meet specific business needs. However, such modifications are discouraged because they can complicate future upgrades and maintenance.References= SAP S/4HANA deployment options documentation

For data migration directly from an SAP ERP system to RISE with SAP S/4HANA Cloud, private edition, the use of Extract, Transform, and Load (ETL) tools is standard. This process involves extracting data from the existing SAP ERP system, transforming it to fit the new data structures of SAP S/4HANA, and then loading it into the SAP S/4HANA environment through a staging database. This method ensures data integrity and compatibility with the new system's architecture.References= This approach is outlined in SAP S/4HANA migration guidelines and supported by tools and best practices provided by SAP for ensuring a smooth transition.