As someone who has now successfully claimed three instances of compensation, two from Air France and one from British Airways, I'd say the AF offer is pretty good. I believe you'd have a very small chance of a successful claim for the delay under the provisions of R261. It appears that despite this, AF recognizes the inconvenience and is being generous. As for the downgrade, you should be compensated for that. Whether you go for the cash or voucher is up to you. I've done both. It mostly depends on your future plans. Just be sure you understand the restrictions on voucher use first. Typically, they're good only on the airline issued it, in this case Air France or KLM.