Change the behaviour of a movie clip

Hi,

just wondering if there is a script that could let me change the way that a movie clip behaves, for example, I have movie clip “a” and in a certain frame I have the function;

a.onRollOver = function (){
gotoAndPlay (“that”);
}

can I later have a script that removes the onRollOver property so that the movie clip is once again JUST a movie clip and not a movie clip button?