Anyone know of a method to do this?

I have tried following this guide:
http://www.unawave.de/installation/d...n.html?lang=EN

And this tool (but I think it just automates the above guide):
http://mp7000.deviantart.com/art/Win...rade-159805048

But each time I just get an error saying you can't upgrade Professional to Home Premium.

The problem is I installed and configured Windows 7 Pro on a new build for my client, I skipped the licence key entry as the client hadn't sent me the COA yet but it arrived today and it's for Home Premium. If I have to I will wipe and start again but the entire machine is configured for them so if there was a simple way of downgrading it would save some time

Cheers