Are Existing Journals Affected After Enabling the Cost Center Requirement?

No. Manual journals saved before you enabled the Require Cost Centers on Manual Journals option aren’t affected. The system checks that required accounts are distributed to cost centers only when a journal is saved, so:

• Existing journals: stay as they are. You can view them in Journal Entries as usual, and no flag or error message appears on them.
• Editing an existing journal: when you edit an existing journal and save it, the requirement applies just like any new journal. If it has a line on a required account without a distribution, the Cost Center is required for message appears, and the change isn’t saved until you distribute the line to a cost center.
• New journals: the requirement applies as soon as you save them.