EC Revenue Manual said:C7. How to calculate on-board Excess Fares
Season Tickets - 7 Day, One Month or longer
Opportunity to buy ticket before boarding the train
The Conditions of Carriage state that season ticket
boarding the train
upgrades must be done before travel, so charge
appropriate fare as though no ticket were held.
However, if the journey is EC-only and the passenger
approaches you before the journey commences, EC
will extend the upgrade facility to on-train.
(Passengers who do not do this and wait for ticket-
inspection staff to approach them should be charged
the full First Single as per NRCoC 38, or move to
Standard accommodation). Charge the difference
between the appropriate Standard and First Class
fares for journey or if cheaper charge the full First
Anytime / Day fare for the journey. No Railcard
However if you board a friendly TOC such as TPE, then it is my understanding that you will be offered the appropriate excess.
In fact, the difference between TPE and FCC is so huge it's hard to believe they're owned by the same parent company at times!