Valuation method and Valuation Rate

Hi all I hav came across one major problem ,
I hope u have Gone through ,
I have created almost 1000 no Items
But now I wanto change-valuation method (FIFO-Moving Average)
But it gaves error-value cannot be changed for Valuation Method
My intention is
If I purchased a one item for 100 RS and give it for further processing by cost 10 RS , Valuation Rate should be 110.

Thanks in Advance

I think you can only do it for new purchase. Old purchase would be on FIFO.