admin can't change guest users name
The administrative view shows a table of all guests, which includes the real name of each such user, as well as the e-mail address, used as a sign-in alias. However, administrators should have the ability to edit the name in addition to merely viewing it. (Currently, they can do so only through the management interface for general users, which is also confusing, because it places guest and regular users under the same management system.)
User ids in Nextcloud are immutable, so this is nothing currently possible. Otherwise https://github.com/nextcloud/guests/issues/333 would be an issue to at least allow setting a custom username.
I believe that you misunderstand the request, @juliushaertl. Currently administrators may change the real name (not the user id), which is not immutable, of a guest user, through the same interface as for regular users. The request is simply to add the ability to make such changes to the table of guest users.