Update on Existing Purchase Order

Hi ,
We have a scenario where we have to update the few columns on an existing purchase order header and line level.

As of now we are using po_document_update_grp.update_document api to update below changes

Line Level Quantity,
Price,
need by date,
promised date,
ship to location.

We have requirement to update below columns as well :-

Header Level
----------------
 Buyer
 Supplier Site
 Payment Terms

Line Level
------------
 Line Type [i.e. Goods or Services]
 Unit Of Measure

Distribution Level
----------------------
Distribution level: Charge A/C



Please let us know if there is any other API (private or public) so that  we can update them  or any other work around to achieve this.