A couple of questions re: changes:

1. With date/time changes, I assume that the both the carrier and routing for a given segment has to remain the same?

Also, I'm currently booked on an AA flight (operated by BA), am I able to switch this to an AA flight operated by AA without any issues?

2. I have a flight that arrives/departs within 24 hours. If I change this to a >24 hour layover, will there be any issues? (presumably taxes will need to be recalculated, so should I wait until the trip commences?)

Cheers