This download from Microsoft indicates that an inplace upgrade from Windows 7 RC to Windows 7 RTM is supported.http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=e170eba1-5bab-401f-bbf5-00f0ee7fe0fb

RC to RTM paths are not supported despite what that preliminary doc states."This document supports a preliminary release of a software product that may be changed substantially prior to final commercial release. This document is provided for informational purposes only and Microsoft makes no warranties, either express or implied, in this document."Carey Frisch

I only brought up the document for discussion since Microsoft published it.