Following the precedent set by CelticFlyer, I'd like to follow up on my own message. I just looked at the definitions set forth in DL's International Contract of Carriage, and I found the follwoing:

"Flight Coupon means a portion of the passenger ticket that indicates particular places between which the coupon is good for carriage."

"Origin means the initial starting place of the journey as shown on the ticket."

So if "No increase will be collected [...] if [...] 2) The originating flight coupon of the ticket is not voluntarily changed at the passenger’s request after a fare increase has taken effect", it should be possible to fly the first coupon (cph-cdg in this case) as booked and then make changes to the entire remaining trip. Suggestions, anyone?