Do you mean CW fares? CE cannot be upgraded as there is nowhere to be upgraded to.

I-class and R-class fares in CW are, as you say, non-refundable, non-changeable and non-upgradeable. However, they can be upgraded with avios post booking with only the difference in fuel surcharge payable. Nicci has confirmed this in the past. Indeed I have managed to do this although it did require a phone call and a fair bit of persuasion to get the agent to look into how to do it.

Edit: This is based on ex-UK fares. I and R class fares from certain other origins can be changed or upgraded for a fee.