Actually, unless something has changed of recent, the upper deck of Air France 747s is SeatPlus, not Premium Economy Class.
Edit to add : but, perhaps, you are referring to the A380. (though, I did not think the OP was flying on an AF A380) Actually, I thought the OP was flying on Delta (DeltaOne class of service) 747s.

Also, I would politely suggest the OP refrain from referring to CDG and first class (with respect to lounges and inflight service) when he/she really means business class.

Air France does have first class (La Première) and business class on many long-haul flights. In addition, there is a dedicated La Première lounge and check-in (VIP fast track) at CDG. (perhaps, one of the best first class lounges in the world).

When the OP says "On the way back from Paris, when we go to CDG, I assume the lounges are the Air France lounges for First Class passengers" - the assumption would be that they are traveling in La Première (not business class); which I do not believe is the case.

As we all know, Delta only has international business class, now referred to as DeltaOne.