During part of the time, yes, It has to install and reboot, after all.
It does save. The messages don't disappear. They just download when your phone is back online.
@dazlicous
But what I’m saying though is what happens when you do a iOS 12 install.
It’s gonna take the iOS 11 completely off and what not.
I think this is more involved than just turning my iPhone off or like putting it in airplane mode.
It’s updating your device. You aren’t losing anything. Once updated your phone is back to normal with a new OS. Messages will appear just fine.
I’m not saying you’ll LOSE the messages. I know they should still be there.
My worry is what if you GET an iMessage like someone sends you one while it’s updating and bc you’re updating you don’t get it.
That’s why I might just do it at 3AM