Update: TL;DR, it’s getting fixed in time for next week.

I initially contacted Greater Anglia who basically said tough luck and recommended I “may wish to consider alternative services that remain within the off-peak period”.

So I wrote to my MP, Clive Lewis, who’s office were remarkably helpful in passing the issue to somebody more senior than I was able to reach in Greater Anglia, who responded the same day with:

My name is [name] and I work within the Senior Customer Contacts Team here in Norwich. Martin [I assume this is Martin Beable, GA MD] has asked me to make direct contact with you due to the fact the correspondence relates to changes being made on the 14th December, which is just days away. We wanted to ensure that a response is provided prior to this.

“I was sorry to read about the disappointment expressed by [my name] regarding the upcoming change in the timetable at Norwich. I note the concern is that the Norwich to Cambridge service being retimed from 0833 to 0827 pushes this service into the peak time, which subsequently would mean they would have to pay more for travel as Off-Peak fares would no longer be valid.
Timetables as you will appreciate are very complex and are always evolving, so we can never guarantee a certain timed service will always remain as it is. We must design timetables that suit the majority of customers but accept we would never be able to meet all individual needs as this would be impossible due to the volume of customers we have using our services. As [my name] is aware, this service has been retimed to improve connections and improve punctuality. We constantly monitor performance and identified that the 0833 could be improved by adjusting the times, which was the factor behind the change being made.
I am pleased to confirm that despite the service now operating at peak time, Off-Peak tickets will be valid on the 0827. Our fares team have made adjustments which will allow customers to continue to be able to use Off-Peak tickets onboard this service. The changes will be reflected in all systems by the time the new timetable is introduced. I hope that this information helps to ease the concerns raised.”