Upgrading from Sitecore 6.5 to 6.6 can raise some issues, more specifically, issues related to the Lucene search module. This module has changed much of its functionality between version 2.3.1 (Sitecore 6.5) and version 22.214.171.124 (Sitecore 6.6). Some of Sitecore’s other modules, like Indexviewer and Weblog, use Lucene as well, so you can run into issues with those modules during your update as well.
The Taxonomy module is an open module whose code can be found in Github. Because of this, you can download the code, change the references to the new Lucene version (126.96.36.199), and replace the .dll generated inside your site’s bin folder.
Even with this workaround, some people still have problems trying to make the update installation package work. This is because there is something else that is not included in the Sitecore update package. This is the new Sitecore.Client.dll. This file has a reference to Lucene’s old version and it has not been replaced in the upgrade process. You’ll need to download the Sitecore CMS+DMS code, go to the bin folder, and replace the Sitecore.Client.dll from 6.6 in your site. After you do this, you can finally move forward in the update process.
Upgrading Sitecore is not a complex process. However, some issues related to modules like Lucene can turn this into a nightmare. If you are planning to upgrade Sitecore 6.5 to Sitecore 6.6, please remember to uninstall all the modules and comment all of your code that has a reference to Lucene before running the upgrade process.
For more information on upgrading your Sitecore site, see Part 1 of this series.