Amend Installment Due Date against sales invoice?

We have a requirement from the business where they want to be able to raise a sales invoice that will use default payment terms. The sales invoice will be completed, printed and accounted. There is occasion where the customer will negotiate a longer payment term that is different to the one stated on the invoice. When this occurs the business would like the option to be able to amend the installment due date against the invoice to reflect the new due date. Is this possible. I have checked and the due date field appears to be locked however is