1. Configure your Active Directory User Directory under Infrastructure -> Directory -> User Directories.
2. Under the partnership settings, on tab 1, scroll down to the section titled "User Directories and Search Order"
3. Move the new Active Directory UD from "Available Directories" to "Selected Directories" by highlighting the User Directory and clicking the Right Arrow ( -> ) Click "Next"
4. Under "Federated Users", where you have configured your LDAP User Directory, on the right side, click "Add Row"
5. Configure your new Active Directory User Directory
6. It is recommended to keep your existing LDAP connection as the first one it searches until you have migrated all of the users over to the new AD connection.