I would not be happy if I had paid the full open fare, and taken a vacant seat with no reservation notice, and then been asked to move.
Given a choice of other seats I would probably move rather than argue.
If standing would result from moving, I would be most reluctant to move.
I would agree with those thoughts.
Should the person requesting the seat have a valid reservation 'chit' for the seat I had occupied (and thus drawn the short straw over) and clearly be less fit than me to stand, I would probably give way - probably expressing my disgust at the system but not "taking it out" on the person demanding the seat.
Section 3.3 of the current National Rail Conditions of Travel says:
'Unless you have made a reservation please note that your Ticket does not automatically entitle you to a seat, and...'
It doesn't actually say "the seat which is stated on the ticket" ... though I appreciate that Advance fares are valid only if the occupy the designated seat. What fun!