Regarding confirmation key change in PO

Hi Experts!

i have to change the confirmation_key (in me23N)

in the confirmation tab. It is working fine if there is no partial gr (entry in delivery schedule tab) against that line item.

But if there is a gr entry against that line item its giving me a message 'confirmation key should not be changed'..

Is there any way you guyz can help me out with that.

thanks