We should be able to update certain fields on an ACCPAY invoice via the API even after the bill is paid. This is in line with the Xero UI. In the Xero UI we can edit the Due Date and Reference still after payment but we can't edit anything via the API. These should match and we should be able to edit the reference (InvoiceNumber) and URL as they do not affect the payment or anything financial on the bill.

For reference the only reason I'm needing to update these fields is because custom metadata can't be applied to invoices so I'm having to store other data in these fields. But still, API functionality should match front end UI functionality