Reinstalling Win 8 at a later date, after Win 7 upgrade

I am going to be doing a clean install of Windows 8 Pro over Windows 7 Ultimate on my laptop. I'll be using the option to burn Win8 to a disk and I have the full Win7 install disk too.

If, at a later date, I wish to do a clean install would I have to install Win7 again and then Win8? Or could I just stick in the Win 8 disk and do it from there?

I know this simple question has probably been asked before but I couldn't find the answer easily. I've never done a in-place upgrade before so I'm not exactly clear how it works in this regard.

Also, out of interest, if you did several upgrades in a row (XP -> Win7 -> Win8) would you have to install them all again in that order if you wanted to reinstall Win8?

Finally, to say thanks for any help here is a cool video: