The JBoss official rules are as follows:
For community projects (i.e. free stuff), we make no guarantees of compatibility between versions.
We will try our best not to break compatibility, or change APIs but there are no guarantees.
For JBoss Enterprise products (i.e paid for stuff) there are strict rules about compatibility and changing APIs. Badkward compatibility is guaranteed between minor releases, APIs are only allowed to change on major releases.