In terms of upgrading, it's just a case of modifying /etc/apt/sources.list to point to the new version of the distribution (in this case that'd mean changing any occurrence of "lenny" to "squeeze") and running "apt-get update" followed by "apt-get dist-upgrade" (both as root). It should be a pretty clean process, as it's usually things like changes to the syntax of config files (in various packages) that cause hassle, and you probably won't have made too many of them (I've only had to think about it at all on a few servers I administer, and only because some of them run a ludicrous amount of services running on them).