Pre-Load before Unloading Original?

[JoelTHE]

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.