SAP S/4 HANA is implemented using SAP Greenfield, which avoids taking over pre-existing system architectures. It's an entirely new system. While there are risks involved, there are also opportunities.
All systems and processes are fully reinstalled and changed when reimplementing SAP S/4HANA, always adhering to SAP best practices and standards. This is analogous to a business that is implementing ERP software for the first time.
To get ready for the switch to the new ERP software, in these situations, the current business procedures and data are first examined and optimized. The master data is moved and the users are educated after the system is operational. After that comes the go-live.