According to the rules you can cancel without penalty and then get a new ticket issued. If the ex-IST price hasn't changed since you booked the ticket then then the only additional cost would be any extra fuel surcharges imposed since then. You do not need to pay a change fee unless you wish to do so in order to avoid a new ticket.

But I think you do need to do this through the IST office. Why on earth did you issue it so far in advance if you weren't sure of the routing?