Originally Posted by corporate-wage-slave

I think this is broadly right, but because BA seemingly has "placed" the traveller on AA, I think that can be construed as a downgrade, but may need some argument on this due to the cancellation factor. If you are cancelled, you have rebooking rights based on comparable conditions, so the next First flight is tonight. The cancellation compensation would be payable if the delay was 2 hours plus at arrival or 1 hours plus earlier departure, that stands alone - it would be best to give the exact times in these sorts of posts to maximise the chance of an accurate response, but I presume this is BA1594, operated by AA, which gets in at 22:10 versus the original 19:45, so unless there is some other factor, the delay compensation is in the back here. So going AA potentially gives more compensation (and certainly more reimbursement), going BA in the evening gets First, so it's a simple choice.

[It also depends on the cause of the cancellation, if it was staff sickness downroute that may be extraordinary circumstances for example]