When I'm done with my exams I'm going to sit down and write upgrade-deblet - the functionality is already there just not nicely done. The idea is that once you have the system installed with the installer, it has installed a local APT repository on the file system, that can be rebuilt with new changes from SVN easily, and you can upgrade just like you would with apt-get upgrade - so you don't have to reinstall by with new fancy package/feature