Do I have to cancel Premium when upgrading to Ultimate?

It depends on how you have booked your premium subscription. 

Premium subscription via Zattoo-Shop:

No, you don't have to cancel your subscription additionally. The Premium subscription is automatically cancelled when you purchase an Ultimate subscription.

Premium subscription via Google Play:

To take advantage of all the benefits of Ultimate, you must first cancel your current Premium subscription via Google Play, otherwise it will continue to run parallel to your new Ultimate subscription. Click here for detailed cancellation instructions. 

Premium subscription via iTunes:

To take advantage of all the benefits of Ultimate, you must first cancel your current Premium subscription via iTunes, otherwise it will continue to run parallel to your new Ultimate subscription. Click here for detailed cancellation instructions. Please note that Zattoo cannot cancel this subscription for you, as we do not have access to your iTunes account.