It’s easy to change the time of your ticket. I haven’t bothered with the app but the email I have for them has a ‘manage ticket’ option where you can change the time.
If you’re getting the tickets online you also need to ‘check in’ before you board the train. I got a return ticket & on closer inspection it had given me train times which were of no use. It’s easy to change them but I’m not sure if you need the times on the ticket to match the train you’re...