R12: Can i update the Invoice amount header if I have already issued/voided payment?

 In 11i, we could update the invoice amount in the header after payment had been issued and voided.  We would reset the invocie back into the workbench, update the distributions to the correct amount and this would allow me to update the invoice amount header.   However, in R12, I cannot update the invoice amount header.   I hav read in some documentation that the header is editable but I cannot seem to get it done.   The invoice has been validated and accounted as well. 

I would like to have this option available as it would prevent unnecessary cancel and rekey invoices.   Does anyone know if this is possible and how?