How to change the costing method from FIFO to Average if the transactions have not yet been done

How to change the costing method , in inventory setup, from FIFO to Average if the transactions have not been done? Users initially had got FIFO costing method setup on Inventory Organizations which is reflecting in items costs. Now the users want the costing method to be changed to average as they want to follow weighted average method.

How can we change the costing method from FIFO to Average now without having to create new inventory organizations?