To edit the customer name in a journal in which a payment has already been applied, the journal should unapply from the payment record.
But in some journals, unable to see any payment record. The associated payments could not be seen as another journal amount had been applied and the payment amount was zero.
So to unapply the payment from the journal, change the status of the journal to ‘Pending Approval. Then the associated journal will be unapplied and the journal will be available for editing in the customer(Name) field.
After updating the customer name, approve and reapply the payments to the journal.