Dynamically change object type?

Ok… Let me try to explain what I’m after. Any help would be great (even some google search terms to try!)

-= Lets say I have two swfs: ‘parent.swf’ and ‘child.swf’ AND MyCustomObject.as that extends MovieClip
-= In child.swf I have a MovieClip physically placed on stage (not using adddChild). Lets call it “ball_mc”
-= child.swf gets loaded into parent.swf

What I want:
-= once child.swf is loaded I want to change ball_mc from a ball_mc:MovieClip to ball_mc:MyCustomObject
-= Then I want to be able to call ball_mc.init() (init() would be a public function in MyCustomObject.as)

Why I want to do this:
-= I dont want to import MyCustomObject.as directly into the child.swf. This is because there will be LOTS of child.swfs (child1.swf - childN.swf) If the .as is imported directly into each of these AND the MyCustomObject.as changes I have to re-publish ALL the child.swfs so that they publish with the new code… Right?

-= whats the best way to go about doing this?? I just want to be able to change the MyCustomObject.as and have the effects show up on all the child.swfs (but not have to re-publish them all…)

Any help woulf be great!