If you upgrade with American using miles or confirmed systemwides, then you’re protected.
Via Traveling Better, if your flight is effected by weather, crew legality, mechanical issues, or even air traffic control — if your flight is delayed just two hours (and in fact, just one hour if you are an AAdvantage elite member or full fare passenger) then you can be protected on a joint venture partner or oneworld airline in the same upgraded class of service if their flight will get you to your destination faster than the next American flight with available inventory will.