Upgrades can be quite complicated. Even when everything is "fully" tested, offline, totally unexpected things come up when it goes on line that require tweeking, and sometimes, even going back to the old version until a new get well date can be established based on issues found.
Usually, you hope for the best and expect it to take a short period of time, but only the inexperienced would allow himself to be nailed down for a time. Probably less than an hour, but several hours is not out of the question.
Sometimes you just gotta throw yourself out there and see. And even when it all works well, it can be expected the users on this board will find things not experienced or witnessed before so it might become a "work in progress" for a period of time. You just expect the unexpected.
Hope it all goes well.