http://www.vistax64.com/vista-installation-setup/134133-upgrade-vista-business-vista-home-basic.html
make sure you have sp1 loaded and run the attached 2 files, then pop in the disk and upgrade
NOTE THAT SERVICE PACK IS VERY IMPORTANT TO VISTA SETUP! Unlike XP, which didn’t care whether you upgraded or downgraded with a repair installation, Vista requires that you install with the same Service Pack on the Disc as the installation being repaired (“upgraded”) and will not allow you to do it otherwise.
Here’s how to upgrade to Vista Business from Vista Home:
1. MAKE A SYSTEM RESTORE POINT. If something goes wrong, you don’t want to be unable to undo your changes.
2. You apply the registry mod, and reboot. You won’t see a visible change yet. The boot screen will still say home. However, if you look at the “System” page in the Control Panel, you’ll see that the key has been changed, and you have “3 days left” before activation throws a fit. Don’t bother trying, it won’t activate. It’s an OEM pre-activated key that only works on the machine it came from. Weirdly, even this page will likely still say “Home” edition. However…….
3. Now when you put the setup disc in, and run Setup from within Windows, Setup will be fooled into thinking that the current installation is Vista Business, and the “Upgrade” option will be available! And, I might also mention, assuming there’s nothing wrong with your installation, if you click on “Upgrade,” it upgrades! That allows you to keep your programs, documents, settings, and DRIVERS. It will take a few hours, but just let it go……if all goes well, you’ll end up with a machine that is exactly as it was, but is now running Vista Business instead of Vista Home.
Please reply if this works for you. I presume most of the people who would try this have moved on to 7 by now, but if you get some use out of this info, please let me know! I spent a lot of time banging my head against the wall to figure this out. I hope it helps someone.