Bazaar has a strong commitment to inter-version compatibility both on disk and over the network. Newer clients should be able to interact with older versions on the server (although perhaps not at optimal speed) and older clients should also be able to communicate with newer versions of Bazaar on the server. Divergences from this rule are considered bugs and are fixed in later versions.
That said, Bazaar is constantly improving and the most recent versions are the most featureful and have better performance. In particular, the Bazaar versions 2.0 and later have significant advantages over earlier versions, including a more compact disk format, faster network operations and overall performance improvements. With the 2.0 release, Bazaar has moved to a stable/development release model where the 2.x series is maintained with bugfixes releases for six months, while simultaneously the 2.(x+1) series is being developed with monthly beta releases that are suitable for everyday use. Bazaar development has a stable trunk with an extensive test suite, so there is no reason to fear using the development series for everyday use, it simply changes more often than the stable series. Many users do run the development version of Bazaar and update it regularly, including most of the Bazaar developers themselves.
Upgrading the Bazaar software is as simple as re-installing the Python package using either the latest binary package for Windows or Mac OS X, the binary package provided by your Linux distribution, or installing from the source release. See http://bazaar-vcs.org/Downloads for the latest releases for all supported platforms.
Bazaar’s later versions support all of the earlier disk formats (back to the very first one), so there is no need to upgrade the branches on the disk when upgrading the software. To make use of particular new features that might need updated versions on both the server and developer’s machines, it does not matter if the clients or the servers are upgraded first.
Disk format upgrades
In its evolution, Bazaar has used a sequence of disk formats for improved storage efficiency and speed. With the new disk format released in version 2.0, there is a commitment to keep that disk format until version 3.0 is released, which has not even been planned yet. (Bazaar 2.0 was released almost two years after Bazaar 1.0.) As a result, disk format upgrades should be extremely infrequent.
If there are existing branches in an older format that you would like to upgrade to the latest format, you can see the 2.0 Upgrade Guide for more information. From the system administration perspective, it is important to coordinate the timing of various upgrades in the process. First, the central branches on the server should be upgraded. Next, any local mirrors that developers have should be upgraded. Finally, developers’ local branches should be upgraded. These upgrades will require an appropriate version of the software whenever they are performed. (It is possible to upgrade branches remotely over the network, but it may be much slower.)
When Bazaar does update its version, plugins that use the Bazaar API may need to be upgraded to reflect changes in that API. Some plugins have strict version dependencies on the version of the Bazaar API that they will accept. If this is the case, then you should ensure that the plugins you depend on have been updated before you upgrade your Bazaar version to avoid a situation where your plugins won’t work with the installed version of Bazaar. If this does happen, then the solution is simply to reinstall the previous version of Bazaar that did work with the plugins. For installations that depend on a large number of plugins, this sort of version upgrade should be tested in a safe sandbox to ensure that the entire collection of Bazaar and its plugins can all work together.