Flightstats shows a variety of aircraft on the DL-operated flights into CDG but it is 767-heavy.
I don't believe I would clamor to spend an extra 20K miles solely to get Business class on a 757 PHL-CDG, although a Business redemption could also get you 1st class (as available) on the domestic segments.
Are you finding Delta's lack of rebooking flexibility domestic vs. TATL, or really Delta-operated vs. AF-operated?