Unload movie with transition, then load new movie

I’ve searched this boards for a “while” but I haven’t found a solution to my dilemma, so…
What I’m trying to accomplish (nothing new, I’ve seen it in a million web sites) is a transition to unload my external .swf files.
Basically, I have a root movie, with a number of buttoms that load every section inside a container.
Currently, when I load a new section, the external .swf in use is replaced by the “called” .swf, but this seems too abrupt to me.
So, how can I unload an external movie with a nice effect or a fading and load a new movie after that? Where should I insert this transition? What should be the actionscript to put in the buttoms?

Hope some can help me.
Thanks.