I just made my second attempt to call and change my preview date, since my work schedule has been changed and I can't go on my original day anymore. Last time, just over a week ago, they told me there weren't any openings, but to keep calling back to see if any cancellations opened up that I could take.
Today, the CM told me that they can't make changes any more on their end and that only the "Ratatouille people" checking guests in for the previews even have access to the reservation data at this point. He said that change happened last Friday (only two days after I was told to just keep calling back on various days, I should note).
He said I could try actually going to Epcot on one of the days when previews were open to see if the CMs at the gate would let me in on a discretionary basis, but that there's nothing that can be done just by calling anymore. He said he doesn't even have access to cancel the preview reservations anymore.