Twenty Ten was the default visual theme for WordPress 3, until it was replaced by the Twenty Eleven theme. About the same time of the release of WordPress 3.3 in December 2011, the Twenty Ten 1.3 upgrade became available. WordPress offers to update TwentyTen in the Updates section of the Dashboard.
If your WordPress site runs on Twenty Ten, you may want to know what’s changed in the theme before applying the update. If, like me, you’ve built a child theme customizing Twenty Ten, you’ll probably be quite anxious to know what’s changed, because some parts of a child theme are made from copying the parent theme’s code and modifying it, so some updates to the parent theme would not automatically flow through.
The ticket for the release of Twenty Ten 1.3 doesn’t really give much info about the changes. Unfortunately there’s no README file or release notes included with the Twenty Ten update, nor could I find any on the Net.
The good news is that the updates to TwentyTen, from version 1.2 to 1.3 are quite minimal. The stylesheet has a line added, there’s simple changes to three PHP files (functions.php, loop-single.php, loop-attachment.php), and a 17 images (mostly header images) are marked as updated. You can see the differences detailed here. There’s a forum thread asking about the 2010 update at WordPress.org.
If you wanted to see the differences for Twenty Eleven, they’re here. A very similar list of files has changed.