This is gonna be a tough one. Technically you're entitled to the difference between business and coach for the return. What that would have cost at the time of booking vs. Full Y can be radically different. Plus getting CO to refund it to you when the ticket has been essentially "used" is going to be another challenge.

In these situations, best bet is to push real hard with the agent who is downgrading you. I know it's tough, especially with the language barrier, but at the end of the day, KLM will want to put you on KL/AF metal as much as possible, but it's possible that BA or DL or someone had space in business. With a bit of pushing, you may not have had to suffer the downgrade.

Also, if the CDG-IAH flight on AF was the problem, was there another way to go CDG-IAH (e.g., CDG-JFK/BOS/IAD/ORD/etc.-IAH on AF/CO?).