I've installed Ubuntu 10.04 6 months ago and then I've added a lot of packages. Now i upgraded to Ubuntu 10.10 and i want to remove all the packages in order to revert to the original ubuntu 10.10 fresh install package-set.How to do that???
Just install 10.10 on the same partition as 10.04 was on before. You'll need to use the "advanced" partitioning in the installer to select the partition (and make sure you don't format it!). The installer will then delete everything on that partition except /home, /root (and probably other user data, I don't know the exact list) before installing. The result is a clean install, but without the loss of your documents etc.
(You might still want to backup your data of course, in case you (or the installer) make an error or something.)
It is virtually impossible, because you cannot say if a package, like
Moreover you cannot say if some new package version has some new dependency, or lack some dependency it previously had (this is not common, anyway).
However, in the /var/log/dpkg.log* files there is all the history of your system, but it is not simple to analyze and take some decisions based on that.
Come to mind that you can do a