Solution : https://service.sap.com/sap/support/notes/987610 (SAP Service marketplace login required)
Key words : 
logistics - general / basic settings / define output format, release ecc-dimp500 installation / update, long material numbers existing, transaction se11/se16, material number length, material numbers, material number, change 'material, default length, transaction omsl_di
Related Notes : 
       
| 854523 | MPN conversion exit is delivered as 'active' by default |