I suspect that AF V class can't be upgraded with miles, but the details of whether this is DL or AF ticket stock and whether it's a DL codeshare (my guess is yes) might matter. You might have more luck using GUCs (if you have some to spare), but IIRC this would only upgrade as far as CDG and not for the (I assume) intraEU flight from CDG to PRG. I don't remember the current rules in detail, but be aware that GUC upgrades versus upgrades with miles can have different rules and require different upgrade inventory classes, although I might be remembering the KLM situation. A further complication could be whether the AF flight has a premium economy cabin or not.

I would certainly assume that these are bulk/consolidator tickets, so that could be a further complication. DL might not be able to touch the ticket before T-24, but in that case I'm surprised that you had a phone agent looking for upgrade space.

IMO you should figure out what the correct rules are and even check for upgrade inventory yourself, then call if you want to use miles or GUCs to upgrade them. If you're going for the upgrade, look at the seat map first too.

Does AF sell cheap upgrades at JFK?