How can I edit an invoice with a "Paid in full" status (currently non-editable) to change the account on the invoice record? The invoice has two payments attached (both with a "Deposited" status and editable) and one attached credit memo (with an "Open" status and non-editable).

not possible, only way to change the GL acct is to reclass it via corrective JE

Oh! got it. Thank you @Sam-I-Am .