I've had a few challenges with the change to the new booking system! There were a few advance tickets that I needed to change, and I was advised to contact the customer services team - presumably employed by TheTrainline. I must be about a dozen e-mails into my conversation with them now, for a simple change of three tickets. At first they wanted to charge me £10 a ticket for each change. I reminded them of their terms and conditions. They advised me to book new tickets and then contact them for refund of the old ones. I did so, and included both the original and replacement tickets in the e-mail. They then asked me for a transaction ID. I referred them to the tickets I had sent them, but they seemed to want something else that wasn't on the tickets. I think the issue is that the transaction IDs for the new website have a different format from the old, and so they are looking for something in a different format. I've replied to explain this, but I expect it'll take a while to get it sorted.
Meanwhile, on the new site, I booked a new advance ticket and then tried to change it to a ticket on another date at the same price. Unfortunately this didn't go smoothly either - when trying it on the website, I was logged out; on the app, I got an error message. I raised it via Twitter (
https://x.com/Graham_P_Martin/status/1979460442272805326) - judging by their Twitter feed, there are a lot of people who are unhappy both with the transition and wiht the new booking system...