Return tickets - change of class in both directions
Simply pay the difference between the price of the ticket you hold and the cost of the appropriate First Class Return ticket for the journey you're making.
I don't think there is a rule against this, as long as the outward journey was made at a time when the First Off-Peak was valid - which might be a little tricky to believe if the ticket has already been used. If the outward journey has not yet been made at all then I don't see why not. If the excess is performed, they wouldn't get anything back.
Thanks. They're traveling next Weds. I've found some EMT ticket office staff to be rather reluctant to do excesses so wondered if this was possible, as they're traveling off peak both ways at non fixed times and would prefer 1st.