When Microsoft made it clear to users who wanted to use the Windows 7 beta and release candidate builds that there would be no upgrade path for them, it also announced that the release candidate would live for a few months past the official street date of Windows 7. That bill is coming due on Monday when the Windows 7 release candidate will start to automatically reboot every two hours. It turns out, though, that there is an unofficial upgrade path for adventurous users who aren't afraid to get their hands a little bit dirty.
It's important to note that because this is an in-place upgrade, the end system that you wind up with could potentially be less stable than just doing a clean install. There are also two ways to do it: one involves upgrading from the RC to Windows 7 Ultimate, the most expensive version. The other requires more tweaking but will support any version of Windows 7.
Before you begin, it's strongly recommended that you back up your data. Once you've done that, disconnect any USB peripherals that are not mission critical to prevent conflicts. They can be reconnected once you're finished.
Next, insert your Windows 7 DVD or USB key into your computer. Do not run the installer, but instead copy the entire contents to your hard drive. You'll need around 8 GB. If you only have access to a disk image of Windows 7, you can extract the contents of the ISO using 7-Zip or IZArc.
From there, you'll need to open in Notepad the file cversion.ini, located in the Sources folder. In Notepad, change the string "MinClient=7223.0" to "MinClient=7000.0" and then Save it and close Notepad. If you're using the RC, you could change it to 7100, but this way you're safe whether you've got the beta or the RC. (Note that the beta started auto-rebooting back in July 2009, so it's unlikely anybody's still using it.)
If you're updating from the RC to Windows 7 Ultimate, you can skip this next step. If you're updating to the Home Premium or Professional version, these next steps are essential. In terms of features, the RC is the same as Ultimate, and while you can upgrade Home Premium or Pro to Ultimate, you can't downgrade Ultimate to a lesser version.
Open the Registry on your installed version of the Release Candidate and open the HKLM\Software\Microsoft\Windows NT\Current Version folder. In the right-hand pane, you should see two Registry keys: EditionID and ProductName. If you're upgrading to Windows 7 Home Premium, both keys should be changed to read "HOMEPREMIUM". Likewise, for Windows 7 Professional, the keys should be changed to "PROFESSIONAL".
From there, just run the installer from the Windows 7 directory you created when you copied the files to your hard drive. When you reach the Upgrade or Custom option, choose Upgrade and follow the instructions. Any programs that you have been running in Compatibility Mode will flag a warning, so you might want to consider uninstalling them until you've finished upgrading. However, this is nothing more than a speed bump, and won't derail the installation.
Although this upgrade path has been well-tested by others, it's important to stress that the process isn't supported by Microsoft.