Core uses a combination of 5 digit-4 digit account numbers. A user wants to use one or the other number system and not a mix, but cannot change the system accounts.
System accounts cannot be edited directly from the Chart of the Accounts screen. So you need to temporarily change the default system accounts under Global Settings and then edit their account numbers in the Chart of Accounts screen. When done, you can re-select them in the Global Settings screen. Check Core Help Center for details.