Windows 7: Will Windows 7 Upgrade work on Windows 7 trial version?

System - Mac w/Lion running Parallels so I can run Windows 7 software
Purchased already - Windows 7 Home Premium full retail version

My old Windows machine was dying and my family wanted to move this last family member PC holdout over to Mac. This is my first experience with Mac. My grown son wanted to make it as easy for me as possible, so he loaded a trial version of Windows 7 on the new Mac and got everything set up in parallels for me. The idea was that after I transferred all my files over to the new machine I could just move the activation over to the new machine and we would trash the ancient PC.

I was all set last night to officially activate the trial version when I discovered that the new machine's trial version is Windows 7 Professional, not Windows 7 Home Premium.

What I need to find out is if I purchase the retail Windows 7 Professional Upgrade version, could I simply enter that product key and not have to uninstall and reinstall now that I have the trail version for pro all nice and settled. I really do not want to have to uninstall, especially with the added complication of Parallels, install Home and then upgrade. I want 'KISS' and simply enter the product key for the professional upgrade and just keep on working with my set up system. Will that work?

To be honest, I don't know whether a Full Retail can be used to Upgrade an unactivated system -but I think it should work.
Back up your data and settings anyhow - using the Windows Easy Transfer process - to external stoage, in case it fails badly.
Make sure that you start the upgrade from within Windows, rather than by booting Windows from the DVD.
If you're lucky, you won't have any problems.


Argh! got it the wrong way around.!

OK - Try again....
You cannot install Home Premium as an Upgrade to Pro - it's a downgrade.
What you could try is using the WET process to export your settings and data to external media, then install the Home Premium in a new Parallells environment, and then import the WET data to that. It should take 90% of the pain out of it, and you still have the trial to fall back on if it goes pear-shaped.