PURCHASE ORDER ITEM CATEGORY NOT CHANGE

Dear all

User have make one purchase order with one line item as ACCOUNT ASSIGNMENT   as K  and item category as null.

Now user has make me2dp  down payment request against that purchase order.

Now user want to change item category as D i.e. service for that purchase order.

System does not allow to change item category. what to do in this case?

Thanks

Nitin