Well I know that if you have to change the ticket after flying 1 segment your screwed. I was flying with my wife to see her family on a Companion certificate in June connecting in DTW. While in the lounge waiting for the connecting flight my wife's knee popped and we had to return home. At first they wanted to charge me 1K to fly us back home which was absurd since we only had flown 25% of the itinerary. In the end after a lot of negotiation they offered if I paid additional $400 and surrendered the certificate they could get us back on the next fight home, oh and with me in coach. Being the discomfort my wife was in I reluctantly accepted.