In the Installation Checklist we state that it is not possible to convert Ops Manager to LDAP authentication after first user has been created.

https://docs.opsmanager.mongodb.com/current/core/installation-checklist/#will-you-use-ldap-for-user-authenticate-to-onprem

Then later on in the docs we provide instructions on how existing installation to LDAP.

https://docs.opsmanager.mongodb.com/current/tutorial/configure-for-ldap-authentication/#using-ldap-from-the-fresh-install-vs-converting-to-ldap