Switching from Ultimate to Business

I know this sounds like a step backwards, but does anyone know what the
easiest way is to switch from an install of Vista Ultimate to Vista Business.

I have a Vaio notebook that is labled "Vista Capable" on a sticker. I
upgraded from XP Pro to Vista Ultimate. For reasons Sony can't explain to
me, the Vista drivers available on their website will ONLY install on Vista
Business. They tell me I need to install Business on this machine if I want
the Sony drivers to work.

Thus I'm wondering if I have to do a complete reinstall of Vista with a
Business license, or if I can just enter a Business license key without doing
a full reinstall.

Thanks.