Thanks for this feedback Rob.
I have already created besides the admin two users. I also stored data for these users on the server raid array with Nextcloud. If I reinstall the usermanager and add the same user accounts again after changing the domain name, can I get access again to the Nextcloud files?
I also found this post by the way: Procedure "Change the FQDN" from manual doesn't work
Would this be an alternative approach?