Testing will be done only for the existing processes which are available in Previous version with the new version.
Results will be compared between Old version & New version. If they are acceptable, then you can proceed doing the live system. If it requires any changes, then those settings (Configuration / Customization / Basis) should be done to make it acceptable or in line with desired process flow.
Apart from that, new features in EHP 6 version over your previous versions can be retrieved from
SAP Solution Browser. It requires S-User ID & Password.