This is such a classic Air France move. Paid extra for duo seats near premium economy, then they swap the aircraft and suddenly you’re dumped into the very last row because those are now the only duo seats left.
What really stings is that they want you to pay again if you want mid cabin seats like you originally had. That just feels wrong. You didn’t downgrade by choice, they changed the plane.
Sadly there’s not much leverage in the moment beyond opening a case, but I’d definitely push for a seat fee refund or voucher. Keep screenshots of your original seat assignment and the new one. Sometimes they’ll refund the seat selection after the flight if you’re persistent.
It’s frustrating because paying for seats should lock in that experience, not just the seat type in the worst possible location. Hopefully they make it right, but Air France customer service can be a grind.