I am not sure, but I think this might be best accomplished using 2 files -- 1 that will be the intro, which, when it's over, loads the other file in it's place. That's how most pre-loaders work (when you get a loading... animation which waits for another file to load, then switched to it.) I am not sure exactly how to do it, but I think you can find a standard preloader script and modify it. Or another expert on here can guide you through the ActionScript.
Andrey