Solution : https://service.sap.com/sap/support/notes/155581 (SAP Service marketplace login required)
Key words : 
additional key words va31, changing existing schedule lines, delivered schedule lines exist, delivered schedule lines, schedule line category, schedule line date, 'environment -> document flow', requested delivery date, scheduling agreement display, schedule lines
Related Notes : 
       
| 204130 | SIS update: Collective note | 
| 149159 | Shift of schedule lines in delivery schedule |