Vista 32-Bit to Windows 7 64-bit

I don't know how this will work for updating Vista to W7. I obviously have to do a clean install of Windows 7 due to them being two different bit versions. I heard rumors that you had to have an OS to even install W7 in the first place. How exactly will that work. And if I can actually just clean install Windows 7, how can I prove I owned Vista (I have the key, but that is also an "upgrade only" disc. Will that work to prove it?)

Thanks. Any help would be great.

(Both Vista and Windows 7 will be upgrade only discs)

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