It is *NOT* safe to change any AF seat on DL.com - even if DL is showing you the seat as changed (which rarely happens), the selection is not communicated to AF.
DLGrkItalNY is correct, the only online way to do it is the following:
- on the very bottom of your DL trip display, you'll find the eTicke-number (under Payment Details)
- copy that number and go to airfrance.us
- under "manage my reservations", click on "other search criteria" and paste the eTicket-Number + your last name
Your TXL-CDG seat can actually be changed but often not online. Only a very knowledgable agent can do that prior to departure-24h if your are in Y. Be careful in TXL, the lounge is in terminal A (which has security at the gates), while many CDG flights depart from terminal D now with central security (that is often packed to the get-no-more). Plan at least 25-35 minutes if you dont have an early/late flight and depart from D.