There is no downtime for any upgrades, minor or major. All upgrades and maintenance are done on one server of a cluster at a time, always leaving 3 other servers running. The same logic applies to all software and security updates. The bottom line, users should never experience any downtime due to maintenance or upgrades. If ever something is large enough that would mandate us to work on all servers of a cluster at the same time (supposing we set up a new cluster and need to migrate things), we would advise your superuser at least one week before.