Renewed contract but Netflix premium has been downgraded to standard with ads

Hi,

I'm in exactly the same situation. Renewed contract but Netflix premium has been downgraded to standard with ads.

I've just phoned retentions who advised me to change the subscription in the app and I won't be charged more, nothing they can do their end.

I've tried this but the Netflix app doesn't give me the option to upgrade. 

Any assistance would be most appreciated, or I'll be cancelling and using black Friday to get a deal with another provider...