To upgrade a Debian Etch (stable) installation to testing Debian Lenny (unstable), you can do the following steps

1. Edit sources.list

   sudo vi /etc/apt/sources.list

   change all occurences of the word etch for lenny, or stable for testing

2. Do the update and upgrade

   sudo apt-get update && apt-get dist-upgrade

That is all that is needed to upgrade Debian Etch to Debian Lenny which is expected to be released fall 2008. The process can be reversed if you would like to revert from Lenny to Etch, just change back your changes in sources.list
One note is that you will need to install a new kernel because the Etch kernel is not automatically updated when perform the upgrade to Lenny.