With SAP S/4HANA Cloud 2105 upgrade, Parallel Accounting is introduced.
There are some pre-requisite steps need to be done before P system upgrade.
Starting with CE2105, actual costing runs will be ledger dependent as part of Universal Parallel Accounting. That's why runs created before CE2105 (which are of course not ledger dependent) will have to be completed before the upgrade. This means that all steps which you normally perform have to be completed. If you miss to do so, the "old" costing run can only be displayed or reversed.
Please also refer to below help portal links to get more detailed information:
Please note below caution mentioned in above link:
"If there are still some materials or activity types for which the closing entries have not been posted successfully, the closing entries must also be reversed for all materials and activity types, and the costing run must be deleted in the former actual costing cockpit, and a new ledger-specific actual costing run can be created."
The "old" actual costing runs have to be completed before the upgrade.
The dependent operations which are required to complete the actual costing run need to be conducted beforehand.
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SAP S/4HANA Cloud 2105