Phoenix maintains backward compatibility across at least two minor releases to allow for no downtime through server-side rolling restarts upon upgrading. See below for details.
Phoenix uses a standard three number versioning schema of the form:
<major version> . <minor version> . <patch version>
For example, 4.2.1 has a major version of 4, a minor version of 2, and a patch version of 1.
Upgrading to a new patch release (i.e. only the patch version has changed) is the simplest case. The jar upgrade may occur in any order: client first or server first, and a mix of clients with different patch release versions is fine.
When upgrading to a new minor release (i.e. the major version is the same, but the minor version has changed), sometimes modifications to the system tables are necessary to either fix a bug or provide a new feature. This upgrade will occur automatically the first time a newly upgraded client connects to the newly upgraded server. It is required that the server-side jar be upgraded first across your entire cluster, before any clients are upgraded. An older client (two minor versions back) will work with a newer server jar when the minor version is different, but not visa versa. In other words, clients do not need to be upgraded in lock step with the server. However, as the server version moves forward, the client version should move forward as well. This allows Phoenix to evolve its client/server protocol while still providing clients sufficient time to upgrade their clients.
As of the 4.3 release, a mix of clients on different minor release versions is supported as well (note that prior releases required all clients to be upgraded at the same time). Another improvement as of the 4.3 release is that an upgrade may be done directly from one minor version to another higher minor version (prior releases required an upgrade to each minor version in between).
Upgrading to a new major release may require downtime as well as potentially the running of a migration script. Additionally, all clients and servers may need to be upgraded at the same time. This will be determined on a release by release basis.
Specific details on issues and their fixes that may impact you may be found here.