How to update unit cost in closed sales order line

Dear Experts,

We have a situation where our users wants to change the unit cost of a closed sales order line. Is there any way to do this.

The full story is; Sales order was created and went through the full cycle until sales update. After the sales update, the Purchasing department realized that there is a over charge during the GRN and the unit cost is incorrect.

They have passed an inventory adjustment to correct the unit cost and now they want to reflect the new unit cost in the sales order line in order to get the correct margin.