According to National rail Advance ticket terms and conditions, you should be able to change Advance tickets until departure (emphasis mine):
Changing the time or date of travel
Changes to time or date of travel must be arranged before departure of the first reserved train printed on the ticket, after which the ticket has no value and a new one must be purchased. You will need to present the ticket(s) and reservation(s) when you request a change.
If the online system does not permit a change, that may be due to software limitations. Under such circumstances, I would contact the TOC where you bought the ticket by phone or online chat to request to change the ticket.
Credit:stackoverflow.com‘
4 Mar, 2024
4 Mar, 2024