Hi Mikael,
I've tried it on runbot. When duplicating or manually creating a new journal, it's now (V11) not possible to select a sequence yourself. Odoo automatically creates a new sequence. So I think the only way now to create this issue is by creating a journal by import and having the field 'sequence' filled with an already linked sequence.
I don't know how law is in your country, but in my country (Netherlands) there is no legislation on sequences in administration for bank accounts. It is of course strongly recommended to have a one2one relation between bank account and journal, but no rules on sequences.
As it is probably not possible to manually change the sequence on a journal, you could check if you can delete the journals if you haven't used them yet, and recreate them manually if you need them. If the journals are already used, I'd advise you to manually create sequences and link them to your journals by export / import.
Good luck!