Has anyone tried upgrading rail ticket to 1st class on the train?

[YogaLite]

Google says it can be done in theory, just wondering whether it's actually possible without paying for a new ticket and what rate might be used.
Is it just paying a difference on a basic tickets or some higher rates are used and whether Railcard would be accepted.

(Obviously assuming there are seats available)

Can I resurrect this thread? DD(17- has railcard) and we I going to be doing Kings Cross to Durham (mid-afternoon midweek in August) and then down to Peterborough on a Friday afternoon.

I'm a GWR (southwest) traveller and the advantages of 1st on that line are minimal these days. If anybody has a good idea how much seatfrog might cost (be it "buy it now" versus bid) I'd love to hear.

Is it normal to be offered an upgrade en route?

I've only used LNER once so I am a bit confused how it all works.

@Madcats put the journey into the app and see. If it’s too far away there may not be a price yet.

I suspect we are staying in Std, but for 2 of us we are currently bidding for £15 each (so £6 on top as an " admin fee").

I've been on LNER once in my life. Do they ever offer upgrades on the train on a weekday?

Does seafrog work on trains during the week?

Can get a weekend or bank holiday upgrade on great Western trains by sitting in first class and paying the set fee (is around £20). Can't do it on weekdays

I did it recently. Just say in 1st class and when the ticket inspector came round I asked to upgrade my ticket. It cost 15.