Marco Prandini wrote:
[...]1) I'd like to find which files of a package have been altered with respect to the original version, in the same way I did with "rpm -V".2) I'd like to instruct "apt-get upgrade" to leave them alone
For 2) you can use dpkg-divert to tell dpkg (the low-level packaging system) to divert installations of a file from the default location to an alternate location. See "man dpkg-divert".
HTH Sumo