Just ran into this. Our interfaces were created originally with just nfs but we just discovered some cifs use as well. I can't go back in and modify it. Unfortunately we already have a lot of production service on the interface. Were putting a work around in place now hoping for something in a later release.
We have the same issue ,Running on Cluster data ontap 3.1 , still not able to modify the data protocol on the lif . Currently cifs is enabled , would like to add nfs also , But dont see any option....Need help..