[Technologist]

As I outlined earlier - it may be easier to take something down totally
rather than do it with the resilience model whilst still on air....
which is how most things are done and the system designed to work normally
(it is now more normal to do an upgrade at 10am with full staffing and people awake
rather than the small hours on the morning with skeleton crew and people trying hard to be awake )

But be sure that any good broadcaster (and NHK is certainly that)
can revert and get some thing on air very rapidly!!!
thats how I worked!!!

[Stooky Bill]

As I outlined earlier - it may be easier to take something  down totally
rather than do it with the  resilience model whilst still on air....
which is how most things are done and the system designed to work normally 
(it is now more normal to do an upgrade at 10am with full staffing and people awake
rather than the small hours on the morning with skeleton crew and people trying hard to be awake )

Yes making changes overnight isn't a great idea unless absolutely necessary. Although in some situations overnight is the best time to do work or practise stuff as there's fewer viewers. 

For example overnight you might occasionally see the BBC's satellite channels have brief interruptions as uplinks are switched, or brief glitches on terrestrial as it's backup is tested. But if all goes well these are breaks of a second or so, nothing like the old days when there was an hour of testcard while the RBS system was tested. 

But having a resilient model like the BBC has means that there's never a reason to take a channel off air completely to do work. I know a few times when a BBC channel has gone off air because of a fault - the power outage in Cardiff a few years ago for example. However I can't remember an occasion when programmes have been interrupted intentionally for engineering work*

That's what resilience means, everything is duplicated: sources, sites, circuit paths, power supplies. There's always an alternative and if there isn't then work doesn't happen. Planned work is quite often postponed because it would mean a loss of resilience or risk a service going off air. 

For example if they're planning work on a circuit carrying one path of a channel, but there's a fault on the other then the answer is to cancel the work, not take the channel off-air


* the only exception to this is the 5 minutes or so they put tones out on the FM radio networks every so often