Cost Adjustment in the middle of financial year to maintain correct cost & valuation in PMAC method

Customer is in EBS R12.2.10 Oracle Process Manufacturing, They had implemented the OPM with PMAC (cost method) in 2011 and posted journal to GL only for 5 months since 2011 to till date. Inventory transactions are there since 2011. They are maintaining cost in excel separately . Now they want to adjust the cost in 2021 in system and want to maintain valuation and proper costing without processing the previous periods data ( which are final accounted but not posted to GL or not accounted for certain data which are in error, and the costs are inaccurate like abnormally negative or