I don't think that EC261 applies.
You are not denied boarding or have a cancelled flight.
You voluntarily change your flight (you could keep your original flight if you wish)..
What worries me is the process itself, and we know that AF is not good with execution. It seems that confirmation of the new booking and voucher are sent within 72 hours. That would suggest that there is uncertainty about confirmed booking for a long time.
newflyer27, how far ahead of your flight did you get the email/sms?