Originally Posted by ijgordon

My guess is that the MIA-LAX segment wasn't available in "I" class, and instead of pricing it up at a potentially much more expensive D or J fare, the system forced the voluntary downgrade into Y. If so, you can monitor the segment for I inventory, and if it's released, you should be able to get AA to confirm you up front.

(It's also possible that segment is completely full in FC, or that it's operated with an equipment type that your fare doesn't allow, although I'm not sure that's really a thing).