There are two issues here, the downgrade, and the delay.

The downgrade is payable and a refund is due. There is information on post 5 of the guide to compensation. The reason for the downgrade is irrelevant, and the downgraded flight is clearly within scope of EC261.

The delay, well as noted the delay is caused by a delay to a non-EU/UK airline for a flight outside the EU/UK. I think until recently the reasonable point has been that this would not be covered by EC261. However, recent case law suggests if part of one ticket and even if the delay relates to the part of the booking outside the EU/UK on a non-EU/UK operated flight, compensation may be due. The relevant 2019 case is linked to below. This was the reverse situation, so CZ out to AUH, and then a delay on the connecting EY flight on to BKK.

https://curia.europa.eu/juris/docume...c=first&part=1
https://simpleflying.com/eu-passenger-rights/

However, I doubt BA/Comair would just pay up for delay compensation if you claim. I suspect you would have to take it to CEDR/MCOL and rely on that case to show you were due something.