Now that OS X and OS X Server both use the same base OS, many people have been asking what the upgrade path looks like to go from 10.6.x or 10.7.x to 10.8.x Server. Charles Edge has a guide on exactly how to do so. His guide very thorough and I would highly suggest following it if you plan on upgrading to 10.8 Server.
Make sure you have backups as well as backups of those backups before proceeding in case something goes awry. Charles suggests that you use a product such as Carbon Copy Cloner to make a backup of your server before upgrading. If anything goes wrong you can always restore it and try another day.Source: krypted.com