Start from scratch vs. building on what you have
A greenfield S/4HANA implementation may seem like the easiest way to get started, as there is no legacy system to deal with and you have complete freedom in redesigning your business processes. A greenfield implementation may look more appealing, it also is costlier due to the fact that everything needs to be rebuilt, even the elements that were still serving important purposes. Leaving your historical data behind is a downside, as such data still contains valuable insights.
With a brownfield approach, your business can keep important aspects of the existing system – with the option of introducing the superior user experience offered by SAP Fiori. This approach is only a fraction of the cost of a greenfield implementation and doesn’t disturb your ERP system, but it has to be done all at once. It’s also important to note that the SAP S/4HANA setup is a bit more rigid, since it follows the framework of your old system.