flash actionscript:

Sounds like you have your terminology mixed up i assume that by .fla you mean a
..swf file running in a browser and that swf file has a button in it and if you
click the button you want the swf to unload and a new swf to be loaded, if this
is what you mean your best bet is to use an fscommand on the on (click) event
of the button that calls to a javascript function inside the html of the page
that spawned the swf and have it load a new swf in it's place.

i hope i interepreted what you were trying to do correctly.