The upgrade history for CLM/ILM has vastly improved with the ILM FP1 SP1 release, with this release the admin can upgrade all versions of CLM to the latest regardless of version.
Prior to this the upgrade could require an uninstallation of the existing installation, depending on which version of CLM you were running.
The current full upgrade package upgrades to ILM FP1 SP1 3.3.1139.02.
See http://support.microsoft.com/kb/977791 for details.
See also http://support.microsoft.com/kb/2346516 for updating to the latest builds of ILM post-upgrade, 3.3.1169.02.