Change Costing Method from FIFO to Average Costing

Hi

I am using Ms. Dynamics Nav 2009 R2 and my costing method followed currently is FIFO for my inventory and I want to change this to Average Costing.

When I try to change it shows the below error: You cannot change Costing Method because there are one or more open ledger entries for this item.

How can I change the Costing Method? Kindly help

Thanks