Here is the blurb from the Virgin website about the above tickets:
"You can make changes to the date or time of your Advance ticket right here on the site. But don’t leave it till the last minute, you need to do it at least two hours before the original departure time that you booked. Just go to My Account to make the change.
If you opted for a Self-Print or m-ticket, you’ll have to change it online. All changes of any kind need to be made by 18:00 the day before the first departure date shown on your ticket.
Switching your Advance ticket to one of equal or greater value, or to another ticket type, is nice and straightforward. But you do need to pay any difference in ticket cost, and a £10 charge per ticket.
The important bit: when you travel, don’t forget to bring your original tickets with you (including any original Self-Print tickets). Your new tickets are only valid with the old ones, so make sure you have them to hand."