Once I asked Turkish Airlines to change the date of return ticket two months! in advance. They said that this is a subject of a change fee that was 100 bucks above the price of a one-way ticket on their website.

I said that I'd rather buy another ticket from a different airline. They cancelled my ticket and promised a $126 refund, which they never paid.

So, basically, it is highly unlikely you will get the same date change on international flight. Or it will be a rip-off.