Differential invoicing transaction has changed; note changes

What causes this issue?

If when entering a differential invoice or a final invoice you change
the type of differential posting to one of the following, the system
performs a complete reassignment:
As invoice
As credit memo
Manual entries are discarded.

System Response

How to fix this error?

Check the invoice items.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.