So, I'm currently trying to change the return journey of an international flight from Europe to the US, bought through Delta. I got through to global ticketing via chat, and was very confused after our conversation. They had said that my original return fare was not available, so that had to upfare me. The booking class (A) was still available, it was just the actual *fare* that wasn't. They did confirm that they were using historical fares, but were telling me that some historical fares are pulled by revenue management, and not available. I've never had this issue before - usually, if the booking class is availability, seasonality restrictions are met, day of week restrictions, etc., then I can rebook for an even exchange. Does this make sense to anyone?

Other thing I was confused about - I found a fare that they were currently selling which fit my itinerary, and should have worked. They said they could ONLY use historical fares, and that those fares weren't valid for changing returns. Is this right? I thought they could force a refaring with current prices at any point? If you change destination, I feel like they have to - and I could just change my destination somewhere else and then back?

I'm honestly very confused at this point - I thought I had figured out at least the basic of fare rules and how they worked regarding changing return dates. But it sounds like I have some pretty big gaps. Could someone help me out?