The chances of a complimentary upgrade, while already infinitesimally improbable, would be even less with short connection times, because you'll likely arrive at the gate after the final upgrade processing has occurred.

In particular, 90 minutes in LAX with immigration, baggage recovery, customs, echeching baggage outside of customs, going back through security to get airside either using the TBIT-T-3 connector make boarding your LAX-MIA flight a bit of a cliffhanger altogether. (See http://www.flyertalk.com/forum/ameri...eb-2016-a.html.)

(The only possibility of a complimentary upgrade might be if your cabin is oversold and they have to roll some passengers forward into any available seats, but they'd likely try to upgrade AAdvantage Concierge Key, Executive Platinum, Platinum Pro, Platinum and Gold, and oneworld equivalents, in that order before seeking others to move up.)