If "at all costs" means "even if it costs $250", the solution seems clear: buy that retail copy of Windows 7 Ultimate.
If it means "but I don't want to spend more than $50 to avoid it" do a clean install of Windows 7 Professional.
I am running Vista Ultimate SP2, and I just did the Windows 7 Upgrade Advisor Test and my machine passed. I would like to install Windows 7 Professional because that is what my school is giving me for free, but I would like to avoid a "clean install" at all costs. I know I can't go straight from Vista Ultimate to Windows 7 Professional without a "clean install," but you can do it from Vista Business to Windows 7 Professional.
To do this, I would need to somehow install Vista Business over my currently-running Ultimate. Assuming I have a legal copy of Vista Business, is there anyway to install it without losing all my files and applications (ie. without doing a "clean install")? If the only way to go from Ultimate to Business is a "clean install" then I might as well go straight to Windows 7.