Scheduled downtime
While ideally there should be no interruption in the system as a result of the changes made, sometimes it's simply not possible to perform big changes without interrupting the system.
When and whether it's sensible to have downtime may depend greatly depending on the system. For example, in its first years of operation, the popular website Stack Overflow (https://stackoverflow.com/) had frequent downtime, initially even every day, where the webpage returned a "down for maintenance" page during the morning hours in Europe. That changed eventually, and now it's rare to see that kind of message.
But that was acceptable in the early stages of the project as the bulk of their users used the site in line with North American hours and it was (and still is) a free website.
Scheduling downtime is always an option, but it's a costly one, so it needs to be designed in a way that minimizes the impact on the operations...