This is a waste of time and money. We did this on a recent flight from Paris to Toronto, paying $108 US extra for seats in the emergency exit row to get more leg room. We did this months in advance of our flight. However when we got to the airport Air France had sold the seats many times over to many, many people. We were denied the seats and were given crowded seats in the middle of two different rows at the back of the plane.
When we complained, we were told the reservations for the extra seats we paid for were good for a year. When I asked for my money back I was told to do it on line.
We met three other couples on the plane who had the same experience except they were told the reservations were good for three months and were not told how to get a refund.
I did go on line and filled out a complex form that was so hard to figure out that I had to call Air France to get some assistance. I had to scan and send my original tickets, my reservation for the seats and boarding cards. Luckily I had them all. After several weeks and no repose from Air France, I telephoned and was told that they were too busy to deal with my claim but that they would try to attend to it in a few weeks. They are either understaffed or exceedingly incompetent.
It is wrong that they do not honour one’s reservations, repeatedly sell these seats to far more people that can occupy them, make it so hard to make a claim and take so long to deal with it. They certainly want your money immediately but take forever to issue refunds caused by their incompetence.