buildbot-worker has backward incompatible changes and requires buildmaster >= 0.9.0b8. buildbot-slave from 0.8.x will work with both 0.8.x and 0.9.x versions of buildmaster, so there is no need to upgrade currently deployed buildbot-slaves during switch from 0.8.x to 0.9.x.
http://docs.buildbot.net/latest/manual/worker-transition.html#buildbot-worker
http://docs.buildbot.net/latest/manual/worker-transition.html#buildbot-worker