I just called Disney ticketing (714/781-4400), and spoke to Jennifer. She said that there is no option there or at the booths to upgrade 6-day park hoppers to 7-day hoppers, and that that option is not in their system. She said there is no Stacy at that office, and that she (Jennifer) worked in the ticket booths before moving to Disney ticketing.
It would be helpful if the people who were successful at getting this upgrade could give us the phone number they called to get the upgrade.
It could be that I was given the 'stock' answer, but I can only report what I was told from the Disney Ticket Office.