Backup all your config files and your data. Than run a apt-get -s dist-upgrade
. That will simulate the upgrade process.
You should see what software it's gonna be installed and upgraded. If upgrade script needs to overwrite some config files it will ask you if you would like to keep original file or to overwrite it.
You can even compare new files with the already installed and than it's easy to decide if you wanna keep the files or overwrite them.
Backup your server just in case and do read this doc: http://www.debian.org/releases/lenny...upgrading.html