How can I edit the Payment Method on Receivables Invoice when it has already been posted to GL?

Summary:

The payment term for the customer's invoice did not default to Automatic Receipts therefor it is not being picked up by the Automatic receipt batch. I tried to apply the solution of adding the Paying Customer name so that I can change the payment method but it only works on invoices that have not already been posted to the GL. When I try this fix on a posted invoice, the name box and site box are not available and I get the following errror when trying to add Paying Customer "Error for Input String "Customer Name""/

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