Loading external symbols using actionscript3.0 and replacing existing symbol in fla file with the same

Thanks Ned! Well..  I am more looking to have the SWF file generated to have the images picked up from my local folder. if the original FLA file does not change, it's fine with me.

On searching more, I could see an approach of creating a FLA with empty movieclip symbols... name every instance of this empty MC on stage. Then the loaded external image is added to every instance of this MC and MC_instance.addChild() for all instances of the MC will change the image and the SWF will have the new image file in it.

refer  http://www.danfergusdesign.com/classfiles/oldClasses/VCB331-richMedia1/exercises/loadingExternalFiles.php

However this means, I need to name all instances and not miss any instance on stage in my actionscript.I need to have empty MC and not one having a image object already... which can be an issue especially in a large FLA file and in future maintenance if I change the FLA file over the period of time. But is there a better way.. can I change the attribute of the MC class inn library itself runtime using actionscript..., so that any instance created from that symbol in library will automatically be applied with that change during SWF creation?