Tim Roll-Pickering wrote:

Why no option to upgrade on an individual journey when using a Season
Ticket?

ISTR that this is (or was) possible, and the charge is the difference
between the Standard Open/Day Single and First Open/Day Single fares,
depending on which is available for that journey (rarely will you find
both). It has to be paid before boarding if there is a facility to do
so, which is a bit of a pain because the only reason I would ever do it
is if I saw Standard to be so busy that it would be seriously
unpleasant to travel therein.

Neil