It is not possible to upgrade 2007 to 2008, because the system itself has changed a lot. If you install the 2008 version, all of your tweaks will be lost, but if you back-up your home directory (/home/<user>) which contains all your program settings, most of them should be saved. Just copy the contents of your back-upped folder to the new one and you should be fine.
Maybe the devs have some folders they would like you to exclude because maybe some configuration files might give errors on 2008, but I'm not sure
