We have discovered a critical bug for upgrade in the 3.0.0, 3.0.1 and 3.1 release. This bug, https://issues.apache.org/jira/browse/CASSANDRA-10822, only affects upgrades from a 2.x version (to any of the currently released 3.x version) and might cause data loss. As such, we strongly advice waiting on the fix for this issue to be released before upgrading to 3.x.
We are currently working on releasing new versions with the fix for that issue addressed. Those will be version 3.0.2 and 3.1.1 and should be released before the end of the week (as soon as we properly tested them). Let me say again that new 3.x clusters are not affected. -- The Cassandra Team
