I have a production server that is running Lenny and was setup following the excellent howto "Virtual Users And Domains With Postfix, Courier And MySQL (Debian Etch) " If I follow the howto "How To Upgrade A Debian Etch System (Server & Desktop) To Debian Lenny" will that blow away all the changes that were made from the previous setup? And if so, is there an equivalent Lenny writeup to put that functionality into Etch?
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/i386/release-notes/ch-upgrading.html