with ref service order

I have created a debit memo with ref to a service order,there are two text line items in the debit memo.then two billing has also happened.now i want to change the plant in the debit memo for 1 line item.

now i have canceled 1 billing document for which i want the change.when i try to change the plant in the change mode plant is greyed out.what shoul i do to change the plant.

if you want further details let me know