Just an update, which leaves me feeling dubious. My email notification of the original flight's cancellation: 17:30 ex MIA after 15:00 international arrival: with its substitution of a 16:43 ex MIA was discussed with BA Customer Service. It wasn't very reassuring that the email gave an incomplete US telephone number (+189062674) but the UK number beneath (+442032500145) got me through. The CS Agent assured me that MCT in MIA is now 1h 35m. That sounds like asking for a miracle, with a guarantee that the bags will be left in MIA, even if we are not. Flight date is July 29, so plenty of time for further changes, I guess. Option offered is 20:01 ex MIA for MCO but five hours in MIA sounds like torture. I think I'll chance it and take (or miss) the 16:43 departure.