I was able to change my Air Serbia ticket dates. I first tried Air Serbia who redirected me to Etihad. Etihad redirected me to trip.com, but they did mention that they can do more if it's in 48 hours before the cancelled flight. Trip.com only offered a refund. I tried Air Serbia again, and while they were supposedly transferring me to a supervisor, they hung up on me. I called again, and the woman I got through to changed my tickets. She did not mention Etihad or trip.com. I don't know if it was a mistake on her part or not. It was very time consuming. The phonecall was 2 hours and 15 minutes, but I got my 7 Lag B'omer tickets changed to Sukkos. The American number for AirSerbia is (718)725-8642.