UpgradeNotes - Community Ubuntu Documentation

An upgrade is the process of going from an earlier version of Ubuntu to a newer version of Ubuntu. An example of this would be going from Ubuntu 7.10 (Gutsy Gibbon) to Ubuntu 8.04 LTS (Hardy Heron). Skipping versions is not advised as it may cause damage to your system. If you are using an earlier version, and want to 'skip' a version, the safest way is to backup your data and do a fresh installation, or to progressively upgrade to each successive version. For example if your are running 7.04 (Feisty Fawn) and want to upgrade to 8.04 LTS (Hardy). You would have to first upgrade to 7.10 (Gutsy), and then again to 8.04 LTS (Hardy) or simply do a clean/fresh install of 8.04 LTS (Hardy). An easy way to test a newer version of Ubuntu on your machine for compatibility is trying the Live CD before upgrading.

UpgradeNotes - Community Ubuntu Documentation