score:4
Ok, so lets establish some facts here.
So, what does this mean for you?
Firstly, the trip is covered under EU261 rules - the journey starts within the EU, this is enough to cover at least the Athens-Belgrade segment of the trip (and if the entire trip is covered on the single ticket, then the entire trip is covered).
Secondly, because the change was made more than 14 days ahead of the start of the journey, you are not due compensation under EU261.
Thirdly, because of the nature of the change, under EU261 you are entitled to a choice between reimburs*m*nt, re-routing or rebooking.
And finally, who needs to fulfil your rights under EU261? The operating airline (item 7 in the preamble) - in your case, this is Air Serbia. EU261 regulations do not deal with ticketing agents, they place all obligations on the operating airline - if the airline adjusts the schedule, cancels the flight, delays the flight or causes you to arrive at your destination more than a set number of hours late, then the airline is obligated to fulfil your rights under EU261.