I would suggest you do as I did in Migrate local users to oauth/oidc .
- Create a new user from the LDAP.
- Check what the table looks like that I posted in the other thread.
- Delete the newly created user.
- Add the LDAP for every user to that table.
All of this is of course done without any warranty as I am not working for Seatable. 
I work in IT though and did worse on production systems when I had to.
Announce a maintenance window, lock it from the outside so none can login, backup the database and update the users.
As far as I can tell the LDAP users have exactly the same settings and rights as local ones, except for the passwords.
