The other slight complication on my ticket was that it's a split-ticket, so effectively 2 advance tickets. That would mean 2 changes - £20 plus any change in fare. I've bitten the bullet and just bought another ticket - this time on LNER website.
All the other ticketing companies (LNER, TrainLine etc.) have websites where you can change existing tickets. Uber does not seem to have this functionality.
The only reason I book with Uber is the 10% cashback. Lesson learned