Upgrading tickets at theatre?

Some places are getting very hot on seat swapping (Old Vic for instance) where additional ticket checks sometimes happen when re-entering the auditorium. I'm old school - sit in the seat you paid for or get an upgrade. I did see a couple recently who had "upgraded" themselves have to do the walk of shame when the people whose seats they had occupied happened to arrive around fifteen minutes late. All a bit awkward and disruptive.

I have moved myself a few times when there were plenty of spare seats further forward, or I've been upgraded on arrival when a theatre has closed the circle due to poor sales. I've only asked when I've had a good reason e.g. a child kicking my seat throughout act 1, or my friend being on crutches and finding it painful to sit in her seat. Otherwise I'd just put up with the seat I paid for.

I know that some theatres will offer a discounted upgrade on the day, but I don't have a list of which ones. That would be useful information.

I wouldn't do this regularly, but when there's obviously a big block of seats free at the same level, I would consider moving. You often get a feel for certain theatres. Moving a few seats along the row to be more central/avoid a head in front etc I'd consider acceptable (although I wouldn't do this to then block the view of someone behind) - you can then easily move back if someone does appear. The only time I've ever really had a problem is when a theatre upgraded a load of people around 15 minutes into a show and moved them all forward. I was one of several people who had moved across, but others had also moved and so blocked the area in between my original and claimed seat. I did go back to my original seat but had to go the long way around. I'm not sure the poor usher ever understood what was happening, or if the people they were moving all ended up with seats in the new area or not.

As I usually buy tickets in pre-sales, I'm usually close to the front and centre, so have decent seats. Although a few weeks back I was at the Lowry, in the second row on the left, by the end of the first act the 4 seats to the right were still empty so moved across as they were more central - slightly annoying someone in the row behind as they were using the seats as a clothes rack.

Over the years I have seen a few occasions were someone has claimed someone else's seat and then refused to move.

So judging by the replies, it doesn't seem like asking for a cheap/ free upgrade at the theatre is a thing that many people do.

Most of the time I'm quite happy with my seat but there are times when I'd only watch a show cause it's cheap. This is usually cause I bring the kids and when you end up buying 3 or 4 tickets, it does add up. Especially if it's for a show that I personally have little interest in.

Varies hugely from theatre to theatre. I booked a cheap seat for 42nd Street, front row but off to the side and flagged as having restricted legroom. Regretted booking it and as soon as I got to London I went down to Drury Lane to ask to move across the aisle. The guy on the box office acted like it was like it was something major, and practically tried to talk me out of it, even though I was willing to pay the difference.

On the other hand when I saw Long Day’s Journey into Night (the Lesley Manville/Jeremy Irons one) I noticed there were unsold seats and the dynamic pricing had moved the pricing bands down, so that better seats were available for the same as I paid for one further back. Went down to Wyndham’s an hour before the start and asked the box office and the guy there let me move, and because I’d booked directly with them he did it for free.