Migrating from user_sql to other authentication, preferably nextcloud internal user db

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Since Nextcloud 23 is getting close to EOL, and no Issues have been closed in user_sql since March, I am looking for a path to migrate my installation from user_sql to a different authentication mechanism. I have around 50 users, so some manual intervention would be possible.

But what is the proper way to remove user_sql and "create" the users in nextclouds internal user database? Ideally the whole process would be transparent to the end users.

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I removed the users one by one from the database, but was unable to re-add the users using the nextcloud GUI (because there were existing files). But it worked when I manually added the users to the oc_users table:
insert into oc_users values ('username', 'Jon Doe', 'cryptic password string i copied from an existing user', 'username');
The user appeared in the GUI immediately and after changing the password in the GUI everything worked properly. All files, calendars, contacts and shares were still in place.