even if the accounting period is open, the Name filed at the line level is unable to be edited. The status of the journal is “Approved for Posting Payments Applied”.
The reason why the user is not able to edit the Customer on the transaction line of the Journal is because it is applied to the Payment transaction.
To be able to resolve this, the user needs to unapply the Journal in the Payment by editing the Payment record and unchecking the checkbox in the Applied to subtab of the Apply tab.
Upon saving the Payment record, the Journal should have a status of Approved for Posting instead of Approved for Posting Payments Applied. Furthermore, the Customer field should not be grayed out when the Journal is edited.