Thanks for everyone's feedback. I'll try to call in later today to see if I can rebook the ticket (which starts in the US) to a different country in Europe which the website or app won't allow me to do. I'm expecting to pay the $300 change fee quoted in the confirmation email, though I'll still ask why the website showed the lower $199 change fee when I made the original booking (as it still does now when I start a completely new booking for pretty much all U.S.-Europe economy light routes and fares I tried). I don't expect the phone agent to provide a really clear answer on that, though, it's just a bit annoying.