You need to check the fare rules of the ticket you are referring to. Comparing to what you know from the US or Europe will get you nowhere. If you can't find the ticket rules, call ET and ask about changing ticket.

Anyway, most likely there will be change fee of USD 100-200 and possible up-fare if the original booking class is no longer available. I believe the new return date needs to be booked immediately but I might be wrong - ask ET if it is possible to cancel but postpone the rebooking of the new date until later. For your information if you do this on a TK ticket, it will be treated as no-show and you need to pay the much higher no-show fee (even if technically didn't no-show for the original flight as you cancelled it before the flight; to avoid this you need to rebook for the new date before your original flight - you can't leave the return date open unless you pay no-show fee ).