Thinking of clean installing

Okay, so, I have a Sony Vaio Laptop which came with Windows 7 Home Premium 64-bit preinstalled. I purchased Windows 7 Ultimate 64-bit Upgrade edition from my university and used Windows Anytime Upgrade to upgrade Home Premium to Ultimate a month ago.

Now, I have decided that it may be best for me to repartition this and clean install Windows 7 Ultimate. I have spoken to Microsoft Tech Support and they told me that I can use the same CD-key to activate Windows 7 more than once as long as it's the same computer, however, trying to Activate using an Upgrade key without actually Upgrading (ie. after a clean install) may not work, however, since I legally own Windows 7 Home Premium, I am entitled to the Ultimate Upgrade regardless.

I saw a thread here (don't remember the exact link), where it said that clean installing from an upgrade cd, then not activating, then installing again over the freshly installed OS would allow you to activate using an Upgrade key.

Can anyone confirm that this still works? That thread I'm talking about was over a year old.

Make your Recovery Disks so you have a path back to HPremium, although you can unlock the Ultimate installer to install HP with your Product Key on COA sticker at any time in the future: Windows 7 Universal Installation Disc - Create

The EULA only requires that HP can't be used as long as it is qualifying you to use Ultimate Upgrade version. You can still move retail Ultimate upgrade to another machine in the future, as long as you have on hand any XP or Vista retail OS to qualify.

The cleanest reinstall you can do is to wipe the HD with Diskpart (zeroes) before reinstall: SSD / HDD : Optimize for Windows ReinstallationIf you do this you'll need to do one of the workarounds in the tutorial Nighthawk posted.

If you boot the DVD to do a clean reinstall while any OS is still on the HD it will see it to pass a flag allowing Upgrade version key, even if you go on to use Custom>Drive Tools to delete, repartition and format the HD.