Thanks for the encouragement to keep persisting. While I never had any luck calling, I did send an email message to [email protected], the Dulles service center. It took me a few days to hear back, but eventually a nice woman wrote me back and we had several exchanges to get things changed. She even called me one day to discuss my options! She said the flights I wanted were showing as "waitlist", meaning the home office would need to approve them. But after about a week, and one followup email to ask for an update, I just received two confirmation emails for my husband and I, for the new flights we requested. My faith in Turkish is now at least partially restored, and I'm looking forward to trying out their business class!