The answer would be to uninstall and reinstall AADConnect I think. If you look at the FAQ here, there's an extract that isn't aimed directly at what you're trying to do but does point the way indirectly:
Q: Can I move the Azure AD Connect database from the local database to a remote SQL Server instance?
Yes, the following steps provide general guidance on how to do this. We are currently working on a more detailed document.
Back up the LocalDB ADSync database. The simplest way to do this is to use SQL Server Management Studio installed on the same machine
as Azure AD Connect. Connect to (localdb).\ADSync, and then back up
the ADSync database.
Restore the ADSync database to your remote SQL Server instance.
Install Azure AD Connect against the existing remote SQL database. The article demonstrates how to migrate to using a local SQL database.
If you are migrating to using a remote SQL database, in step 5 of the
process you must also enter an existing service account that the
Windows Sync service will run as. This sync engine service account is
Use an existing service account: By default, Azure AD Connect uses a virtual service account for the synchronization services to use. If
you use a remote SQL Server instance or use a proxy that requires
authentication, use a managed service account or a service account in
the domain, and know the password. In those cases, enter the account
to use. Make sure that users who are running the installation are
system administrators in SQL so that login credentials for the service
account can be created. For more information, see Azure AD Connect
accounts and permissions.
With the latest build, provisioning the database can now be performed out of band by the SQL administrator and then installed by
the Azure AD Connect administrator with database owner rights. For
more information, see Install Azure AD Connect by using SQL delegated
To keep things simple, we recommend that users who install Azure AD
Connect be system administrators in SQL. However, with recent builds
you can now use delegated SQL administrators, as described in Install
Azure AD Connect using SQL delegated administrator permissions.
Of course this rather depends on the uninstall working nicely and that a reinstall won't pick up previous settings and I'm afraid I haven't got a way of testing that for you.