Problem in Changing Stock Revaluation Method

Hi,

One of my client wants to update Evaluation method from FIFO to Standard, Now we have issued all the stock items but from them a few items are not displaying in change valuation window , On checking I found Item Valuation field under Inventory data is still not Highlighted . I had manually closed all pending Delivery Document

Please suggest How Can i update these items valuation method.

Thanks,

Sudhir