Migration is a big topic, but let’s see if we can introduce the subject in a simple way for now. For SAP customers already running ECC on-premise (but not yet on HANA) who want to stay on-premise, here is the path to S/4HANA:
The journey to S/4HANA begins with a careful analysis of the business scenarios to address first, and for that effort, SAP offers a Value Identification Workshop.
The first technical step is to bring ECC 6.x up to Enhancement Pack 7 if this is not already done. Then, if not using Unicode data types, this must also be implemented. At this point, the database is ready to be migrated to HANA. After the migration to HANA is complete, then comes the Simplification Packages, previously called “innovation exchanges.” This is the new code that will bring the benefits you’ve read about for S/4HANA—a simplified data model, for example. The first package that is available is Simple Finance, followed in late 2015 by Simple Logistics. A promotional license is available at no cost to upgrade to the S/4HANA Foundation and includes Simple Finance Accounting and Financial Close.
Once you’ve migrated to HANA (step 3), then as each part of the S/4HANA suite is made available, simply repeat step 4 after acquiring the necessary licensing.
At each step of the journey, there are service and support options available from both SAP and SAP partners.