Dear Clara,
Extended classic Purchase Order can only be changed from SRM side. In standard, if you change the accounting information in backend ECC it will not be reflected on the already created EC PO.
To solve the issue on a non-standard way, you might think about a BAdI implementation (DOC_CHANGE_BADI) which checks the PO version and - in case of a Change version - validates the accounting information.
Hope I could help.
Imre
