Pre-Load before Unloading Original?

I may need a little education if I'm asking this question improperly, so don't hesitate if it's necessary:

On many sites, you click a link in the menu and although a transition animation or preloader animation plays, the original MC continues to play, and the new animation is on top of it.

I have a menu where buttons will load external MCs into a main display area, this I am ok with. BUT, I need the new MC to be preloaded on top of the current one, and only after it loads will the original one be unloaded. There will be a large number (50) of different MCs to load in any order. How do I do this?

The new BMW M5 site does this in a simple way...

www.bmw.com/m5


Thanks for your help.

Use 2 container clips, one on top of the other. Allways load the new movie on the top one. When it's loaded, unload the bottom one (which holds the previous movie loaded), and with swapdepths(), swap the top container with the bottom one, thus making the now empty top one available to load the next movie.

Ok, that makes sense...I wasn't aware of the swapdepths function...BUT, how do I make the buttons know which MC to load the new one into?

Do I set up some sort of boolean test to see which is empty? If so, what does that test look like code-wise?

Can you give some more specific info in terms of the variables involved and how to call/use the swapdepths() function?

Thanks again, this has already been a big help. Looking good is really in the details.