Hey fellow travelers,
Just an fyi based on my recent experience. I booked tickets on Delta from Portland, OR to Paris, France with a connection in Minneapolis. The Minneapolis to Paris flight was on Air France via code share with Delta. I always pay the extra when available on long haul for economy comfort (helps to have that bit of extra space, especially with arthritis pain). No matter how I tried; on-line at both Delta and Air France sites, calling both, etc..... I could NOT upgrade my seat (there were plenty of these seats showing available on-line) for the Minneapolis to Paris leg via Air France.
The Air France rep told me that they do not allow anyone who books on another site (Delta in my case) to upgrade unless there is something left when you ARRIVE at the airport. He explained this was due to Air France wanting to save those seats for their own customers. I confirmed this by calling Delta a second time and it is true. Funny thing is I usually always fly direct to Amsterdam and then take a hopper from there but my son convinced me to try connecting in the US this time. Oh, well. Lesson learned. I realize it isn't the end of the world and bottom line, we will just be happy to get there but I was totally surprised and just wanted to share for those who care about this issue. The Air France rep also told me if I had booked with them direct instead of Delta, I could have chosen and paid for the upgraded seat.
Thanks to all on this site for the wonderful advice, looking forward to our trip. Happy Travels!