Override class method with dynamic function

Does anyone know if it’s possible to override a custom class method with a function expression? For instance, if I want to change the destination of a button on the fly, I could theoretically type:

 var new_destination:Function = function(evt:Event):void{ go_to_new_place();};
myButtonClass.mouse_clicked = new_destination;

but unfortunately that generates an error because the class method “mouse_clicked” is already defined in my class (with the eventListener tied to it). I tried using the “override” keyword in the Function Expression but that didn’t work either. Grrr.