those days) and I regularly upgraded to First - if I had time I would
do it at the ticket office or if the train was busy I would sit in
first and pay the excess fare - definately only the difference between

You can do this if you have a normal ticket, but not a season ticket. If
you have a season ticket you are treated as a fare-dodger and they will
likely withdraw the season ticket at the same time for inappropriate use.

The conditions of carriage state the ticket upgrade must be purchased
before the journey begins if you have a season.

Regards, Dave