I worked on broadcast integration for years. With any serious broadcaster there is plenty of redundancy built-in and every equipment (that can be) is synchronized to a common timing source (black burst back then, tri-level and PTP today).
Since most (usually all) signals are easily routable any distruptions can be kept to a minimum. Upgrades and updates are always taking place, you won't notice when Master Control is upgrading it's playout system, since they route to the signal from the backup system (without a glitch since they are synced) while they perform the upgrade and testing.
Upgrades to equipment that isn't in 24/7 use is easier. You can upgrade equipment for a studio when it's not in use or the programs that usually use it will move temporarily to another studio or do outside broadcasts.
