Solution : https://service.sap.com/sap/support/notes/993615 (SAP Service marketplace login required)
Key words : 
alm bapi downgrade kit, -existent maintenance planner group, maintenance planner group, bapi_alm_order_maintain bapi, system saves, terms plangroup, ingrp reason, program error, solution implement, attached corrections
Related Notes : 
       
| 708508 | Downgrade BAPI_ALM_ORDER_MAINTAIN -> errors |