This is direct from the National Rail service website:
"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."
Quite clear you will be paying the current rate plus 10 if you change it!