Solution : https://service.sap.com/sap/support/notes/490499 (SAP Service marketplace login required)
Key words :
'business area' field, business area, field attributes, longer change, master data, internal orders, entered budget, terms sapmkauf, coas-gsber, screen 315k_object_data_exist
Related Notes :
| 511833 | Enter business area into order posted to |
| 507457 | WBS element with budget:Business area not changeable |
| 356041 | Business area no longer changeable after planning |