Unable to change valuation class even with zero stock.

Hi Guys,

         I wanted to change valuation class of a material. I made the stock of the said material to zero. I have one PO for which MIGO and MIRO has been done. Now when I go to change the valuation class, I am not allowed by the system. System shows me the same PO number for which MIGO and MIRO had been done. MIGO and MIRO has been done for the full quantity and payment has not been made.

        My PO is closed I suppose. Why am I getting this error..?

Regards,

SK