Flash MX 2004.
instance.libraryItem
Property; a library item used to instantiate this instance. You can change this property only to another library item of the same type (that is, you cannot set a symbol instance to refer to a bitmap). See library object.
The following example changes the selected symbol to refer to the first item in the library:
fl.getDocumentDOM().selection[0].libraryItem = fl.getDocumentDOM().library.items[0];