Build messages processor as well as main TeamCity server can be stopped or restarted while builds are running there. If agents can't connect to build messages processor for some time, they will re-route their data to main server. If main server is also unavailable, agents will keep their data and re-send it once servers re-appear.
Upgrade sequence should be the following:
- start upgrade on the main TeamCity server as usual
- at some point you will be warned that there is build messages processor running and using the same database; you'll need to shutdown it first
- proceed with upgrade
- make sure everything is ok, agents are connecting etc (since messages processor is not available anymore, agents will re-route date to the main server)
- upgrade software on the messages processor machine to the same version
- start messages processor and check that it is connected via Nodes configuration page on the main server
At the moment build messages processor node does not support plugins.