I just had a weird couple of tickets. ORD-DTW-LEX on NW and LEX-ATL-MSY on DL. When I was returning home I arrived in LEX early enough to catch a standby flight to DTW. I took the flight because I would rather kill a couple of hours in the World Club than sit in LEX. I had no problem changing the first flight and taking the same DTW-ORD flight that I was originally scheduled on. BTW my DL flights posted at the same time as my NW flights (They we two different tickets)