Yes it is possible to amend the Supply ref, the Invoice Number and the Pay date of a Purchase invoice if the invoice has not been delivered from the system. Also you should consider before making these changes if it is appopriate for your internal accounts process, especially if you have exported the purchase invoice to your accounts system:
Make the changes to the date then click Recreate Document to recompile it. Next click View Document to ensure the changes have been applied correctly.
If the invoice has been delivered from the system, the Invoice Delivered box shown above will be checked and the Invoice Date fields will be disabled. This can be overriden by removing the tick and the invoice will be updated ready to send/resend to your supplier as appropriate.