Why the PO account assignment category cannot be changed

A buyer or requisitioner tries to change the account assignment on a PO line, either the category itself (K to F, or U to K) or just the cost object underneath it, and gets an error such as the field being display-only in change mode, or a message stating the item cannot be changed because of existing follow-on documents. The complaint usually comes in as 'I need to move this PO to a different cost centre / WBS element and the system will not let me' or 'the account assignment tab is greyed out even though the PO is still open'. It is frequently reported alongside a goods receipt or invoice that was already posted, but the requester rarely connects the two events themselves.

Pull the PO history (EKBE) for the item and show non-zero quantity or value on movement type 101 or invoice category lines; that quantity is the reason the category is locked. Cross-check the account assignment segment (EKKN) for multiple distribution lines and flag which ones already carry posted value. If Funds Management is in scope, pull the commitment line tied to the PO item and show it is still open. Attach the exact error message number from the change attempt as corroboration.

If PO history is the cause, this is a data fix, not configuration: reverse the goods receipt (MIGO, movement reversal) or cancel the invoice (MR8M) first, change the account assignment, then re-post. This has real accounting consequences and should go through the same approval as any GR/invoice reversal. If an earmarked funds document or commitment is the cause, reduce or close that funds document in FM before the account assignment field unlocks; also a data-level action. If the lock comes from field selection tied to release status, and the business genuinely needs the category editable at that status, that is a configuration change to the field selection key or release strategy setup and requires a transport plus testing of the release workflow; it should not be applied as a one-off unlock. If the rejection is an invalid item category / account assignment category combination, the combination is disallowed by design; the correct action is to cancel and recreate the line with the right item category, not to force the combination through configuration. If the item is linked to a sales order, the account assignment cannot be changed at all on that PO line; the sales order or the PO line itself has to be cancelled and rebuilt.

The common reflex is to edit the account assignment segment directly through a table editor, or to force the field open with a screen variant, treating the lock as a UI nuisance. This bypasses the very check that protects the posted GR/IR and CO postings, and leaves the PO's value string, the material document, and the cost object out of sync with each other. A second reflex is deleting and re-adding the same line to sidestep the lock, which breaks the PO history link to any receipt or invoice already posted against the old line number and complicates GR/IR clearing at period end.

Purchasing raises the ticket but the diagnosis sits with MM functional support, pulling in FI-CO or FM ownership whenever a commitment or posted value is the blocker. The handover note should carry the PO number and item, the account assignment change requested, the PO history status, the exact error message, and whether a funds commitment or invoice is attached, so the receiving team does not have to re-run the same checks.