Follow-up:

Some seats opened up in the earlier Air France flight from CDG to Spain leaving at ~3pm. I noticed this about 2.5 hours before that flight's departure. I went back to the AF Customer Service desk and a helpful female rep said that she thought she should be able to retrieve my suitcase from where it's being stored for the latest flight and bring it to my earlier if I manage to be booked/re-booked on the earlier flight 'soon'. This alone made my day better: I felt like at last someone was trying to work with me and help and offer options.

I called Flying Blue right away -no wait this time-. However, as other folks just said on this thread, the friendly Flying Blue rep stated that once I already took the first segment, Flying Blue could neither change nor cancel the second segment: the segment is part of a 'whole ticket' that cannot be priced or changed separately. My only option at that point was to buy a new ticket CDG-Spain. They were selling it for 24k miles + ~$70. I was ready to pull the trigger but when I clicked, the price changed to 70k miles, which is more than I was ready to pay to save me 5 hours of wait.