You can't change the costing method if item entries exist. You will have to set up a new item with a new no. to take the stock.
Run the Item Cost -Adjustment ( to bring the average unit cost up to date), make a note of this unit cost ( if SKUs are used take the unit cost from the SKU card) or copy it temporarily to a new field in the item table. Then post a negative adjustment with an item journal to bring the inventory to zero. Copy the item into a new item, set the new costing method and post a positive adjustment using the unit cost noted earlier. All this has to be done separately for each location
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Run the Item Cost -Adjustment ( to bring the average unit cost up to date), make a note of this unit cost ( if SKUs are used take the unit cost from the SKU card) or copy it temporarily to a new field in the item table. Then post a negative adjustment with an item journal to bring the inventory to zero. Copy the item into a new item, set the new costing method and post a positive adjustment using the unit cost noted earlier. All this has to be done separately for each location