calculation type change in header condition of shipment cost

Hello

       Could anyone tell if we can change the calculation type in header condition of shipment cost(VT01N  or VT02N)?

User said they can change it before, but now it is greyout. why? 

      P.S. the calculation type is T, we need the header condition value apply to all items. but it seems only the calculation

type changed to B, it will apply to all items.

Thanks.

Sophie