BAPI_PO_CHANGE doesn't change MATNR

Hi experts,

I'm using BAPI_PO_CHANGE to modify the material in several PO but it doesn't work. If I modify it using ME22N it works fine, but I need to do it massively and so I can't use ME22N.

The code I'm using is:

MOVE gwa_data-ebelp TO gwa_poitem-po_item.

CONCATENATE lv_matnr 'N' INTO gwa_poitem-material. "lv_matnr is the old material

APPEND gwa_poitem TO gt_poitem.

MOVE gwa_data-ebelp TO gwa_poitemx-po_item.
MOVE 'X' TO gwa_poitemx-po_itemx.
MOVE 'X' TO gwa_poitemx-material.
APPEND gwa_poitemx TO gt_poitemx.

CALL FUNCTION 'BAPI_PO_CHANGE'
         EXPORTING
           purchaseorder = gwa_data-ebeln
         TABLES
           return = gt_return
           poitem = gt_poitem
           poitemx = gt_poitemx.

The messages I get are:

But nothing is changed in the PO.

Thanks for your help.

Carles