Debian: Upgrade Etch to Lenny, and error message “There is no public key available for the following key IDs: xxxxxxxxxxxxxxxx”

It’s pretty easy to upgrade your Etch to Lenny. Open your repo list and replace all references to “etch” with “lenny”. To edit your sources.list:
nano /etc/apt/sources.list

Then update your repositories:
aptitude update

and proceed to upgrade:
aptitude dist-upgrade

I ran into an error when I was updating my repositories: “There is no public key available for the following key IDs: xxxxxxxxxxxxxxxx”. To fix that error:
gpg --recv-key --keyserver wwwkeys.eu.pgp.net xxxxxxxxxxxxxxxx
gpg --export xxxxxxxxxxxxxxxx | apt-key add -

Replace xxxxxxxxxxxxxxxx with the key you see in the error message and run aptitude update again.