According to this article:
Can I upgrade from one of the Windows 8 previews Microsoft has shipped this year?
In follow-up answers, Microsoft said that users running Windows 8
Release Preview -- the newer of the two sneak peeks, shipped on May 31
-- can upgrade using the Windows.com upgrade tool and pay the $40.
However, the only bits that are retained during that upgrade are
personal data files. Other elements, including the applications,
Windows settings and user accounts that migrate from Windows 7, do
So, for anyone running Windows 8 Release Candidate, the "upgrade" is
only as effective as that from ... wait for it ... the 11-year-old
Windows XP, which also keeps only personal files. Go figure.
Although Microsoft did not spell out what options are available to
those who stuck with 2011's Developer Preview or the February 2012
Consumer Preview, our assumption is that such an upgrade won't be
There is also an unsupported method to upgrade from the RP to Pro/Enterprise RTM:
Extract the Windows 8 ISO or copy all files off the Windows 8 DVD to your HDD.
Browse to the Sources folder and open the cversion.ini file.
Find the entries shown below and change the values from 8508.0 to something lower, such as 7100.0:
Start the RTM installation process as usual and select the Upgrade option.
According to some sources, this unsupported method even lets you keep all your files and applications, but that's not something I can confirm ATM. In any case, it is always good to have an up-to-date backup of all your personal data on another physical drive before you attempt an OS upgrade/install.