Wrong Opportunity Tied to an Invoice

Hello,

I have an invoice that was accidentally created from the wrong opportunity.  In the recent past I have been able to edit an invoice and change the opportunity as needed.  I've noticed now that on the invoice, the link to the opportunity is inline, even when I edit the invoice.  In the past, the invoice would show the link, but when editing it would display as a drop-down automatically where I could select a different opportunity.  I do have a custom invoice form using the standard opportunity field which is set to normal.  This is such a basic thing, I