While the fare rules and t&c for MCE are that it is status on the day of travel that matters, AA does not do regular sweeps and will therefore not ordinarily move you out of MCE.
However, as noted, once you are no longer Plat., do not make any changes. That includes seat assignments and SDC, not to mention changes, without understanding that your new seat will not be in unpaid MCE.
The only unfortunate uncontrollable situation is an aircraft swap, IRROPS rebooking or other involuntary change.