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?