Cannot change valuation method for Item

Hi All

I am having this issue at at client where I cannot change the Valuation method from FIFO to Standard for a certain item.

All the stock was issued out and open PO's closed.

It happens on the customer side they on SAP8.82 PL04,

I upgraded their database to SAP8.82 PL07 and it still happens.

I tried this on another Item and it works fine on another item that is setup simiilarly.

Is there something I am missing?

Thank you

Jerusha