w7 upgrade ???

i read on microsofts website that you can buy an upgrade version of w7 for 50$ i was wondering if you can upgrade from the rc cd or if you have to reinstall vista or xp then upgrade to the new w7??

As with all previous versions, you must have a previous version disk, not the RC release.
So XP, or Vista disks will work fine, you do not need to install that OS prior to installing the upgrade of 7.
It should ask you for the other media to check that is legit.

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