The best and easiest solution for this is.
Inside FM "ISU_MRRESULT_DEMARCATE" we have one more FM "ISU_DB_V_EDISCOBJ_SELECT_LNR_H" where system checks whether this contract is disconnected or not if disconnected then it will update variable "y_disconnected" as "X" so at the end of "ISU_DB_V_EDISCOBJ_SELECT_LNR_H" FM SAP provided one enhancement space their based on your requirement you can Change "y_disconnected" value as initial so system will do proration as it is non-disconnected installation.