Below is an email recieved from Microsoft when the RC was realeased.
Please note: All users of the Windows 7 Release Candidate (including Windows Vista users who have upgraded to the Release Candidate) must do a clean installation of Windows 7 RTM. Please keep this is mind as you consider downloading the Release Candidate as opposed to waiting for the general availability release.
Thanks again for your help in reaching this important milestone. We hope that you will join us in testing the RC and finishing Windows 7!
As far as I know nothing has changed so you would need to do a clean install as I see it. This would mean buying a full copy of Windows 7 and not just an upgrade package.