If you'll miss a connection, you can usually get rebooked on pretty much whatever is available on AA, including codeshares. I've had this happen even before I was elite, getting on an otherwise more expensive nonstop because of a potential misconnect. They won't require the same fare class to be available. Maybe it was $150 more when you purchased, but the rebooking agent doesn't necessarily know that. I've been able to do this even if it just looks like I'll miss the connection, but the retimed flight doesn't violate MCT. Though I don't know the rules in that case.

I've also had luck with this in advance. A schedule change on MAD-PHL-BOS caused me to voluntarily accept an overnight layover (same day was available via JFK but I didn't like the schedule). But when space on another previously oversold PHL-BOS became available for same day connection, I got rebooked onto that, even though the cost of the standalone PHL-BOS segment at the time was nearly what I had paid BOS-MAD r/t.