After you’ve backed up your AppleTV disk, the next obvious step is to enable SSH so that you can connect to the AppleTV remotely. This will let you connect to the machine and make updates without needing to remove the hard disk anymore.
Tutorial Ninjas put together a nice howto for doing just this. It walks you through disabling the firewall, enabling the ssh daemon, and disabling the Watchdog app after rebooting so that you can run other applications –Link.
Once you have SSH working, you can replace your hard drive and boot the AppleTV. The userame and password for ssh access are both frontrow, and that user has sudo access (also with password frontrow). Keep in mind the security implications here.
If you want to lock down ssh and set things up to use RSA authentication, you’ll need to install ssh2. awkwardTV has a wiki that explains how to swap out your ssh1 installation. Just be sure that you can connect to the new ssh daemon before logging out or you’ll have to connect the hard disk to another machine again –Link.