Sourced from here..
If you apply the preview to an existing Windows 8 installation, when it comes time to apply the RTM, assuming that you still want 8.1, you will have to re-install all apps and desktop programs after the RTM is applied..
.. EVEN IF you roll back to Windows 8 before applying it..
If you wait for the 8.1 RTM, as I am going to do, you can install it without having to re-install all of your other stuff again. As eager as I am to see it running, I don’t feel the need to have to back up and re-install everything when everything is working so well.