Howtoforge posted a guide about upgrading Debian Lenny (Debian 5.0) to Squeeze (Debian 6.0) on Xen VPS

This tutorial shows how to upgrade a Debian Lenny (Debian 5.0) installation on a Xen based Virtual Private Server (VPS) to Squeeze (Debian 6.0) including kernel update, dependency based boot sequencing and conversion to UUIDs. If you do it the usual Debian way just with apt-get dist-upgrade you will most likely end up with an unbootable system. This is mainly because the update of grub fails.

  How To Upgrade Debian Lenny (Debian 5.0) To Squeeze (Debian 6.0) On Xen VPS