This is the problem and something I encountered over my trip this summer. The Delta web site will show you all the available seats that have not been assigned for the Air France flight that you can pick, at least if you have status. The problem is that the seat selection on the Delta web site does not make it to the Air France systems for confirmation.

Do not change your original seat choice as you will essentially guarantee that you will lose your seat assignment. The Air France web site may not actually allow you change to an open seat either. It will either result in some generic error message meaning your seat assignment will be lost or all seats will be shown as unavailable so you can't change them to begin with.

I recently had the stupid idea of changing seats to a couple of rows forward and closer to the exit 2 weeks before a return flight. That change resulted in losing the original seat assignment and all my family members getting the middle seat in different aisles as opposed to having a whole row to ourselves.

The excuse from the Air France GAs will be "equipment change" even if your on an identical A380 as originally scheduled. Delta will give you an insultingly small amount of miles and no answers.

I suggest booking directly with Air France or not changing your seat assignment ever.