My experience of op upgrades is very very limited and I guess that confirms AF 'revenue preservation' policy

• 1 upgrade to Alizé class (AF's enhanced eco class to Caribbean destinations). My first upgrade ever! Upper deck of a B747-200! I was Fréquence Plus Bleu and checked in very early (3 hours ahead).
• 2 weeks ago on a european route, upgrade from Tempo Challenge (Y) to l'Espace Affaires (C), which is quite unusual since they can adjust the limits of each "cabin" thanks to the curtains. At last, I discovered what happened beyond that mysterious curtain... well, quite disappointing: same snack and same Champagne Gauthier as in Tempo Challenge (full fare economy), preflight drink. And they forgot the hot towel!
• upgrade from Tempo (restricted economy, cheap 125 € ticket booked in N) to Tempo challenge. This one was more interesting, as there really is a difference in snacks between Tempo (1 small sandwich) and Tempo Challenge (cold snack with mediterranean salad, flying camembert , bread and pastry or fruit salad. Champagne Gauthier. This was a strange situation as I must have been upgraded by the KLM staff at Schiphol, whereas 1 south-american passenger seating one row behind me got very embarassed explanations from a FA explaining they ran out of food for him. I guess I was eating his meal. I didn't deserve at all this upgrade on such a cheap ticket...

There are also regularly reported downgrade situations, and I have witnessed one myself... CDG-PVG, 22:30 at the lounge, 1 ex-business class passenger shouting his disappointment to be downgraded in front of embarrassed lounge attendants and tens of other elite+ and J/F PAX waiting in the lounge... a 11-hour night flight on a typical business route, go figure...

I wonder if your odds of upgrade vary from time of the day: CDG-JFK has 7 flights a day (AF/DL) + 3 to EWR (AF/CO). Do you get better chances to be upgraded at 18:55 (AF008) than on the 08:25 flight (AF022) which has more business passengers? It must depend on the type of aircraft too and the respective capacity of each cabin: J/F seats represent 21% of the total capacity of a B777-200 OTC, 19% of an A340 OTC, 18% of a B747-400.