Generally speaking, upgrading from a pre-release edition of Windows 7 (e.g. RC 7100) requires a clean install of the system, meaning that you will need to manually reload some files from a backup or other source, and will lose various system settings.
Fortunately, there is a way to bypass this requirement, although you may not want to for compatibility and stability purposes.
The process, as described on this MSDN blog, is as follows:
- Insert the final-release Windows 7 CD when you purchase it in October
- Copy the whole image to a storage location you wish to run the upgrade from (a bootable flash drive or a directory on any partition on the machine running the pre-release build).
Browse to the sources directory.
- Open the file cversion.ini in a text editor like Notepad.
- Modify the MinClient build number to a value lower than the down-level build. For example, change 7100 to 7000 (pictured below).
- Save the file in place with the same name.
- Run setup like you would normally from this modified copy of the image and the version check will be bypassed.