Sage 100 is a perpetual inventory system, not periodic inventory system.  Each transaction, as it happens, properly debits and credits the inventory and debits and credits cost of goods sold.  Inventory item GL Accounts come from the item product line.

If the GL Accounts in the Operating Expense Area have GL Postings in history, you will not be able to change that from Operating Expense to Cost of Sales.  Instead, create a new GL Account in the Cost of Sales and then journalize the ending balance from the old account to the new account.