A new implementation has a clean core by default. However, even in a system conversion, getting (and keeping) the core as clean as possible is recommended. How do you do this? By utilizing SAP’s recommended workstreams.
Once the technical conversion is done, SAP recommends that project implementation teams use four workstreams to proceed with the migration, in accordance with the clean core approach:
Evaluate custom code using the ABAP call monitor and the Custom Code Migration app.
Adapt custom code automatically using the Quick Fix functionality as part of ABAP development tools for Eclipse, or manually in those instances where automatic adaptation is not feasible.
Review legacy modifications to determine those that are obsolete, unused, irrelevant, or replaceable.
Optimize all code that will be retained after the conversion using ABAP tools such as the SQL monitor, SQL Trace, and the Performance Monitor.