I'm not sure that this comment is relevant, but last March I used AA miles to book award travel BWI-JFK-TXL-STR with the first flight on AA and the others on AB. My BWI-JFK flight was delayed for several hours, so the connection at JFK became impossible. After consultation with a supervisor, the very helpful AA agent at BWI offered to put me on BA flights BWI-LHR-STR with no extra charge. I chose a different option, for various reasons, but the offer was made.