newbie question on finding and keeping customized files with dpkg or apt

[Marco Prandini <prandini@ideafutura.com>]

Hello,
I'm switching to Debian after a long time on RedHat, and I haven't been 
able to find a couple of functions of the package manager I'd like to 
use... hoping they exist at all!

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

That's because I didn't resist the urge to make some customization to my 
system, and I don't want them to be overwritten by the upgrade procedure.

Thanks a lot!
Marco