Changing Allocation Cost Element in KP26

Hi All,

I am trying to change the allocation cost element in KP26 , for accomplishing this I am changing the allocation cost element in activity master and then in KP26 trying to delete the line corresponding to the old allocation cost element but I am unable to do so.

Can you please suggest a way to change the allocation cost element in KP26 or to have new allocation cost element into effect with the plan activity and price ?