Please let me know if this is the way to do this - or if not, what I will need to do to switch over to the public repository.
There is no way to unregister from ULN from the client. You need to delete the server from the web interface. However, you can stop a client from using ULN without unregistering it. Just run:
# yum remove yum-rhn-plugin
That removes the ULN functionality from the server. Now it will use whatever yum repos you've configured, which if you grab the public-yum-ol6.repo will be the public-yum server. You can then remove the server from the ULN interface at your leisure.