I would not expect a fare / tax re-assessment to occur when being involuntarily reaccommodated onto another flight and that is my experience with AA

The assertion in post 2 that they would have charged the surcharges in the reverse situation is false; AA will re-accommodate onto BA for no charge when an involuntary rerouting occurs which I have direct experience of

I would not suggest trying to make a "test case" on a flawed premise

Writing to AA , explaining the situation and asking whether they will do something may produce a result