Clear purchase order match information for an invoice that has been posted

The advice here would be to raise credits for the affected transactions - matching them as required - and re-post the transactions to allow a redo of the match.
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It will not be possible to change the Invoice, but it may be possible to amend the order and so if you would like to discuss this then our support team will need to investigate, run scripts or liaise with the hosting or development teams. Any Issues with contacting us, please see the following Get support in Access Financials.