Yep that’s an annoying thing in nextcloud. I’m also searching for a convenient solution to switch from ldap to local auth.
Maybe this will help you: Import LDAP users & get rid of LDAP
I haven’t tried it yet but I surely will sometime…!
I think there’s no way without loosing passwords…
But yea there’s no easy way at the moment. I wish we had a functionality like in Gitea, where you could just switch the auth backend as I like.