When a new major version of WordPress is released, such as WordPress 2.1 or 2.2, you may find that your Themes and Plugins no longer work the way you are used to. It could be that they are completely broken, because something in WordPress that was fundamental to them changed, but it's also possible that you can get them working again by updating some settings, or maybe just a small edit.
This article is part of a series on Migrating Plugins and Themes. It covers changes between WordPress Version 2.5 and Version 2.6, and what you need to do to Plugins and Themes to get them working in Version 2.6.
If you are the author of your Theme or Plugin, or if you have modified or customized your Theme, then this article will help you upgrade your Theme/Plugin for 2.6. If your Theme or Plugin is distributed publicly, after getting it working you may want to add it to the Compatibility lists above, so your users will know it is working and which version they should be running.
The header of the Admin section changed, and there was a new hook called 'sidemenu' that you could use to add a link to the links on the right side. However, this went away in WordPress 2.7, so it is not recommended that you use it!
There is more flexibility about where your uploads, plugins, and language files should go. Some useful constants:
There are also new options called "upload_path" and "upload_url_path", which give the path and URL to the uploads directory.
For more information and sample code for use in plugins see Determining Plugin and Content Directories.