The goal of the system conversion is (as the name implies) to convert one existing on prem SAP system to an on prem S/4HANA system. The major reason for choosing this path certainly is to keep the existing implementation in the sense of conserving the value of it i. e. the money that was already spent in order to make things running as they are. However, adoptions will have to be made and this conversion should also be seen as a chance to do some bookkeeping and get rid of "old" stuff that no longer adds value and adopt to the innovations of S/4HANA. As being on S/4HANA at the end of the process the adoptions of the new features can be done in an iterative approach.
Having said that, let us take a closer look what steps in detail have to be made in order to walk down this conversion path.
The starting point is an existing Business Suite system. The release you have to start with is a ERP 6.0. the EHP does not really matter i. e. EHP 0-8 are valid starting points when converting to S/4HANA Enterprise Edition 1511. What is important to mention is that it is not necessary to have the ERP running on a SAP HANA database at the start of the conversion. The migration of the database can be integrated into the conversion itself. So let us assume that we are on an anyDB system.
The conversion then basically consists of three steps that are executed in the so-called realization phase:
1. Database Migration: Migrate to SAP HANA as database
2. Software Update: Update the system to contain the new applications as well as the new DB tables
3. Data Conversion: Convert your existing table content into the new S/4 data model
All three steps can be executed together in a row.
The database migration is a "usual" database migration using the Database Migration Option (DMO) of SUM. The only thing worth to mention is that the system has to be a unicode system when switching to SAP HANA (as well as when upgrading to ERP 6.0 EHP8).
The software update as well as conversion are supported by the Software Update Manager (SUM). This tool tackles all the relevant steps of the so called process. Actually some tasks (especially in the later phases of the conversion e. g. some customizing activities) are manual tasks. They are planned to become fully automated in upcoming releases (probably in release 1610)
Before starting the realization phase some pre-work has to be done in order to make the transition a success (or to be more precise to get the system running after the migration). This is quite obvious as you know that SAP did several ground-breaking changes in S/4HANA and this has impact on the code (yes there are compatibility views, but have you ever tried to write to a CDS View. I think the technical term when doing that is … KABOOM).
So before you start the conversion you to run through the so called prepare phase. In this phase you will use the maintenance planner in order to run the relevant pre-checks. This comprises a check of your code against the so called simplification list. This list contains all the technical simplifications that are embedded into S/4HANA. So in the preparation phase you have to compare the custom code and the simplification list in order to extract the spots in your custom code where an adoption is necessary. SAP offers the custom code analysis tool to do that. As this step is a crucial one when it comes to the necessary conversion effort you do not need to have a S/4HANA system up and running to do so. You can extract your custom code via the custom code extractor from your ERP system and import it with the custom code analysis tool into a SAP NetWeaver 750 system (yes just a blank NetWeaver is needed). It is not unusual that customers use the cloud appliance library (https://cal.sap.com/) and create a temporary instance of a NetWeaver in the AWS or MS Azure cloud for that check in order to save time and money. As a result, you get the Custom Code Migration Worklist which is a list of all findings that need to be adopted. Each item in the list is also connected to a SAP note that describe the necessary task to resolve the finding. Be aware that these finding are necessary for a functional correctness of your system. There are no hints concerning optimizing the performance (usually part of the pre-work for the HANA migration)
That is basically it, so the overall process of the conversion looks like this:
To make the story complete: If you have been an early adopter of the Simple Finance Add-On (which is basically an Add-On on the ERP) you can also move to S/4HANA Enterprise Edition. Both currently available releases (1503 and 1605) are valid starting points. It is also possible to switch from the ERP system just to the Simple Finance (or S/4HANA Finance as it is called now), but this is not recommended.
So now you should have a deeper insight into the necessary steps you have to take on the road to S/4HANA.