This might be of interest to you, then.
When I got on the train I found my reserved seat was marked on the overhead whatnot as being reserved. So far so good. It also said that the seat next to mine was unreserved. Also fine, because I I was travelling alone. When we left my station, the display changed for my seat to say it was "Unreserved if unoccupied". The status for the seat next to mine still showed that it was unreserved. This is different from in before times where the display for the seat reservation stayed reading the same for the duration of the journey. So if I got on in at Preston, without a reservation and I spied a seat marked as reserved from Lancaster to London and there was no one in that seat, I would sit in it, because I figured that whoever should have been in it from Lancaster had obviously not shown up. Or had met someone they knew and sat with them instead.
I would say the new thing of changing the status of the seat once the station has been passed is because they know that in cases where two people are traveling together, while the online system doesn't let you buy two seats together, you both have to reserve a seat. So in all such cases, the seat that you or your other half isn't sat in will get a change of status on the display. That's my take on it, anyway.