I am surprised for a multi tenant application that as far as I searched and dug there is not easy way to do this.
You either gotta move the entire installation to a larger server
Upgrade the existing one if possible
Or…
You/user move manually by creating a new account for them and either having them move their data over or having the admin do it for them (which is a obvious issue as the admin should never have to interface the actual user’s GUI).
Which all are not ideal, I don’t understand why you can’t just move user A from server A to server B assuming both have NextCloud installed and configured…