I don't think that if you allowed someone to 'upgrade' on the train that they'd suddenly opt to just reserve any train to get a cheap fare and then upgrade on the train, or claim a refund on the advanced ticket later (with or without an admin fee).
The 'upgrade' is simply too expensive, but lessens the blow should you find yourself having had your meeting cancelled and you now want to go home now instead of in a few hours time, or a meeting overran and you missed your train by a matter of seconds. It doesn't have to be a business meeting, in case we assume that businessmen and women will always buy a full price ticket - which isn't the case, as staff here are told to get advanced tickets, even if they can't be sure when they need to come back.
Given you should expect that, except for exceptional circumstances, you will have your ticket checked* - what possible advantage do you have in buying the advanced ticket? If you think you won't be checked, why get a ticket at all? Or simply buy a totally invalid ticket (YPRC, Disabled railcard etc) and claim ignorance. (* As you will on a train that has advanced/reserved seating).
Even your seat reservations are invalid, so you've gained nothing over simply paying the full price on the train anyway.
And I can change my advance ticket on a low cost airline - but so far have never done so. I recently wanted to take an earlier flight home from Dublin, with seats available, on Aer Lingus and it came to about £500 from a ticket that was originally about £60.
That was with an admin charge AND the difference between the last-minute fare. Funnily enough, I didn't do it - and with airline security and boarding procedure, I couldn't just go ahead and do it anyway as I could on a train.
So, while people will be outraged at having to pay extra to travel earlier (and Aer Lingus could even sell our seats on the flight I'd no longer take two hours later) it won't interest the media as nobody actually got made to pay it. On the railway, you could just say 'f**k it' and board anyway - and then you've got the Daily Mail story when caught.