Understanding version numbers
The first part of each Zen version number shows what version of Drupal core that it is compatible with. 7.x-* versions work with Drupal 7. 6.x-* versions work with Drupal 6. Etc.
The second part of each Zen version number represents the major and minor version numbers. 6.x-2.* means it is the second major version of Zen. 7.x-3.* means it is the third major version of Zen.
Versions of zen with the same major number will have nearly identical markup, CSS and PHP and updating from one to another is considered a minor upgrade.
Versions of Zen with different major numbers will have significant differences between the two and updating from one to another is considered a major upgrade.
The markup and CSS between minor versions of Zen (when the last digit of the version number changes) will always remain the same, except when a bug fix requires a change. Any bugfix that requires a modification to your sub-theme will be carefully documented.
Examples of minor upgrades:
Upgrading 6.x-2.0-beta1 to 6.x-2.0
Upgrading 6.x-1.0 to 6.x-1.1
Please see the documentation listed below for minor upgrade instructions.
We try to make upgrading Zen as easy as possible, but upgrading a custom theme in Drupal is naturally one of the hardest things when upgrading a Drupal website. Unlike modules, you can't just download the new version and be done. Zen sub-themes hook into the markup and CSS from Drupal core and from the base Zen theme. So when those change, your sub-theme can render completely differently than it used to.
Examples of major upgrades:
Upgrading 6.x-1.1 to 6.x-2.0
Upgrading 5.x-1.1 to 6.x-1.1
Please see the documentation listed below for major upgrade instructions.