Windows 8 Download / Pro Upgrade

Looks like the option to buy the upgrade for $40 is live for me... but I have a few questions.

I plan to upgrade my HTPC and my home desktop.

The HTPC is already running Windows 8 Consumer Preview. I would like to do a clean install on this one. I do have a Win7 Ultimate License (what the machine was using before the installing the preview). I should be able to run the update assistant in on the 8 preview install right? Then what happens? Do I choose clean install and will it let me burn a disc or something? When I install will I use my Win7 key? How does that work exactly? What happens to the Windows 7 key, is is "migrated" to Windows 8 and no longer valid for 7?

For the desktop I would ideally be able to buy now for the discounted price. Can I run the assistant, buy, download and just hang onto it for a little while?

Anyone got experience doing either of these?