As detailed on the National Rail Enquiries page for Advanced Purchase tickets:
Advance fares are valid only on the date and train shown on the ticket and are non-refundable. You can change the time or date of travel before departure of the first reserved train.
Any difference in fare and an administration fee will be payable.
Based on that, I would've expected to be able to pay the standard admin fee of a tenner (£10) up to the time of the train's departure, plus any additional cost for a different Advanced Purchase ticket (eg original ticket £30, new one I'd want to change to £25, pay £5 more including the admin fee)
This morning, I had an Advance Purchase ticket booked, which I wanted to postpone to either the afternoon, or a few weeks time. However, when I went onto the website I'd bought the ticket from (which is also the website of the people operating the train), it wouldn't let me make any changes to it - the dropdown box for the AP ticket lacked the details that a subsequent Anytime ticket did have
As the whole reason for me needing to postpone it was that I was unexpectedly busy, heading to the station before the train wasn't an option. I don't therefore know if it was a website issue, or I'd misunderstood the change option on the ticket.
So, for an Advanced Purchase ticket, under what conditions can you change to a different AP ticket, how, and until when?
Whilst there are no refunds on Advance tickets in most cases you can change your journey prior to travel (up to departure) for a £10 fee plus any difference in the fare payable. However, please note that if you choose (where offered) to print your tickets at home or download them to your mobile device, different rules apply and you may not be able to subsequently change your ticket. The relevant conditions will be made clear when buying your ticket.