Thanks for the quick answer. I've already did some research on this subject but still have several concerns:
1. What if the flight on the new date costs more that my original flight ? Do I need to pay the difference between the price of the flights?
2. For now it seems that the price i'm getting using star alliance is significantly higher that using multiple one ways. I tried to optimize my route to direct flights but still big difference. what are the coupons you mentioned ?