Find out more information here.
There is also supposed to be some sort of gift given to Zagreb ticket holders who exchange their tickets for the Belgrade show. I have no idea what the gift could be. Chances are, neither does Real World or LiveNation. You'll find out when you get there.
It is important that all tickets for the Belgrade show are exchanged for tickets to the Zagreb show. You can start doing this tomorrow, 9 September. I could not find much information on whether refunds will be offered for those who do not want to go to Belgrade. I assume refunds are being offered, but I wouldn't bet the farm on it.