Instances on stage (exported SWF) is undead in ActionScript?

1

Say I have an instance on the stage in Flash CS3 (assume a designer put it there) and give me the SWF.

Here is my problem:


// suppose the linkage name is "someMC" and 
// the instance name is specified as "instance"

// Get a MC from my library:
someMC = new SomeMC();
addChild(someMC);


// It's easy to use it with ActionScript code
someMC.instance.alpha = .5;


// Work, just clear the instance!
someMC.removeChild(someMC.instance);

// seems to be working.
delete someMC.instance; 

// Will print null - seems everything is okay.
trace(someMC.instance) 

// -> ERROR here! saying something that I 
// cannot change the variable type of "someMC.instance"
// - while I have already deleted it already, WHY?
someMC.instance = new SimpleButton();