Intercompany with open ledger entries

Hello,
We would like to start intercompany invoicing between 2 companies in the database. The customer and vendor numbers are already in use for a long time, and there are open ledger entries. So NAV does not allow to use them as IC partner.
Can anyone explain me why?
The only transactions we want to do is a sales invoices becomes a purchase invoice in the second company.
I can force the use of the customer/vendor no, but I wouldn't like to break functionality.
We are using BC14

Thx,
Andy