Now, with the release of Windows 8 Release Preview, you also have the choice of an in-place upgrade.
Just start up Windows 8 DP/CP, pop in your disc of the downloaded copy of the Release Preview, click on the prompt that will appear in the upper right corner of your screen and click on the "Run exe.", then follow the prompts.
The only drawback of this method is, when you get to the "Install Screen" you're presented with this;
Link Removed
which one can only assume, it means just that, you would have to re-install any programs/apps that you've had on the DP/CP.
Basically, it seems to be an in-place clean install and clicking on "Change selection" will take you to the previous window, where "Nothing" is checked and you are not able...