Twice this has happened to me now: my flight is cancelled short notice (fewer than six hours), BA offers me a flight later that day, but downgrades from CE to ET. In the first instance, I changed to the next day, but checking Expertflyer later I saw seats in CE on the first option BA gave me, but the agent said she could not change it because 'I accepted the change' to the next day flight.
For today's flight, they did the same--downgrade from CE to ET on a later flight, but there were CE seats available. This time I called in and the agent was able to put me in CE. Lesson learned.

Is downgrading a default option when BA rebooks? I want to think not, since it does not happen on my TATL flights--or is that because I am usually in F?