Windows 7 Professional to Windows 7 Home Premium?
I want to change my OS from Windows 7 Ultimate edition to Windows 7 Professional edition without formatting. How can it be done?
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I echo the reformat and reinstall for all the reasons others have noted. I'd only add that you can backup pretty much all your settings and data using the File and Settings Transfer Wizard (I don't think it was renamed in 7). This makes such a transfer incredibly painless.
Just create the archive of all the files and settings before you wipe.
Then reinstall your apps.
Then run the wizard again to put everything back in place. It catches most application settings as well and so there's very little reconfiguration you need to do.
I'd personally backup and format for the installation of Win 7 Professional.
If you want to preserve applications that are installed as well as settings etc - there are some registry keys you can modify to make your system look like a lower version so you can "upgrade" to Professional. I helped a neighbor do this before when a "Tech Friend" installed a bootlegged copy of Win 7 Ultimate.
I used this guide to see what registry keys needed to be modified: