Most of the times the software/hardware vendors allow you to carry out upgrades/updates on the fly. But we know what all might go wrong during it. There's no perfect upgrade procedure that is applicable for all the hardware/software. to be on the safer side you will always have to take a maintenance so that there is no load on the servers and users are not affected during the process. Usually all the vendors don't explicitly mention about it but its common sense like you mentioned to have a window where even if something goes wrong the users will not be affected.
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