Q: Can customers upgrade to First Class?
A: In most cases, yes. However;
1) Remember that changes to travel plans can only be done at a station before departure of the first booked train, so once on board, either the ticket is valid on that train, or it is not.
2) Where the customer joins the correct train and wishes to upgrade to First Class on board, or wishes to upgrade at a station beforehand between Standard and First Class on the correct train on which their Advance ticket has been reserved, this is not classed as a change of travel plans.
Weekdays: All TOCs allow upgrade to First Class in the above circumstances. As with other ticket types, charge the difference between the fare already paid and the appropriate First Single (dedicated or inter-available) fare for the journey. No £10 fee should be charged for doing this (except on XC trains)