Accounting principles require that accounting journal entry modifications are kept to a minimum once an FI document has been posted in SAP S/4HANA. In any case, all accounting related information, like posting keys, account numbers, posting date, amounts, and so on, can never be modified under any circumstance as this data has already led to an update of account balances.
Additionally, it is possible to customize SAP to apply certain rules so that the changes to the aforementioned fields are only allowed under certain conditions. For example, the changes are accepted only if the posting period is open or if the line item has not been reconciled yet. The reason for this is to prevent undesired modifications of data that would make reconciliation impossible or could jeopardize the auditability of the data.
Let's take a look at the change concept in SAP.