Kudos to Martin Brinkmann at ghacks.net. His September 2, 2013, article “How to install Windows 8.1 RTM on a Windows 8.1 Preview system” explains how to get around the MS-imposed restriction on Windows 8.1 installs. Otherwise, MS requires either a clean install of that RTM release (or the upcoming GA release scheduled for mid-October), or a return to plain-vanilla Windows 8 (another clean install) before installing the upgrade bits on any PC with the Windows 8.1 Preview release already installed.
The secret comes form a set of tools called the WinISOUtils, a set of Windows ISO disc image utilities designed to operate on the dowloadable ISO images that MS uses nowadays to distribute its various operating systems. The self-description blurb for this nice little set of tools provides a great overview for its components, so I reproduce it here:
The cversion tool can be grabbed on its own, and essentially sidesteps the version check portion of a typical Win8.1 install by deleting the version info file from the install image in use.
In fact, this technique works for any modern version of Windows, from Vista on forward, where the cversion.ini file is used to check whether or not a chosen upgrade path is allowed or not during the install process. Because it permits users to retain existing data and settings, as well as additional programs already installed, it’s a real time-saver for those not afraid to risk the occasional (and inevitable) compatibility hiccups that Microsoft’s policies are designed to avoid (but which are too draconian to be acceptable to most power users and Windows-savvy IT professionals willing to do their own application-level troubleshooting).