If the IT architecture surrounding MCE is the same as other seat assignments, you will likely keep your MCE seats so long as there are no changes whatsoever. This includes no voluntary changes initiated by you and no invol changes such as aircraft substitution initiated by AA.

It's also unclear what AA's policy would be. If you do make a change and the system highlights your lack of eliginilty, will AA allow its front line people to simply override the system?