It's not as simple as going to a ticket office.

It's not just the £10 fee but also the fare difference,

Advance tickets

If you have an Advance ticket and need to change the day or time that you are travelling, you can amend your ticket right up to the time your journey is due to begin. You may be charged £10 to change each ticket and you will also be charged for any difference in price between your ticket and the new ticket.

Advance tickets are non-refundable, unless your train is delayed or cancelled and you choose not to travel.

See the Advance ticket terms and conditions for full information.