The situation : Payment was confirmed within the wrong journal but the bank reconciliation was not yet conducted. 

The solution : 

For this payment (on its payment form), you will click on the smart button "Invoices". You will find the different invoices linked to this payment (one or more). For each invoices linked to this payment, you will select it, open it and scroll down to the bottom of the invoice and click on the little green button "Information". By clicking on it, you will be able to cancel the link in the system between this invoice and the payment. Do the same for each invoices. 

Once done, you can come back on the payment form of your payment and bring back the state of the document to draft. There, you can modify the journal and restart the reconciliation with the good journal.