I have a policy of only installing Ubuntu's LTS releases, so with the release of Lucid Lynx it was time to migrate from Hardy. I started with an aging Compaq Evo 510s. I used the upgrade path via the update-manager.
Things went smoothly aside from a couple of warnings regarding the flashplugin-nonfree package. I dismissed these, however, at the completion of the upgrade the warning became an error and the upgrade process exited before clean-up and completion.
Upon rebooting the Evo appeared to have Lucid installed but it was very unstable (a separate issue). Fortunately, the Evo is little more than a (XDMCP) thin-client. So, I burned a Lucid ISO and performed a fresh installation. This time the installation completed successfully.