How do they collect the charge?

(1) if you paid with a cc, they could try to bill the cc--I'm not sure that would work. [if pax paid with cash, what happens?...]
(2) they could call the police (it depends on the destination) to arrest the passenger
(3) I've been on many flights where pax have been upgraded by fa's after the plane left the gate--poacher could claim that he/she thought he had been upgraded

They should boot the poacher from the wrong seat IMMEDIATELY.

Don't.

Yeah, this works all the time. What's the problem? If you self-upgrade, that'll save you that many more miles you can sell to strangers on the Internet. Because that works all the time, too.

...

One time I was on standby after taking a VDB voucher, and the only available seat on the next flight was in F. I was the last person to board, and was given the F seat (and also the voucher good for future travel).

I have seen FAs randomly invite economy passengers to move up to premium cabins if there is space, but only on Asian carriers and not recently. I think this is a good practice: costs the airline nothing (don't give them the meal or other stuff, just seat) and results in excellent odds of repeat business.

The answer to your question, by my estimation, is that you have about a 5% chance of being successful. If you don't mind the likely embarrassment of being caught, then you have nothing to lose. They can't legally bill you for it later.

^^^^

On what planet is this the law? You stole a product sold to others at a high price. You can, and should, be charged, and then prosecuted.

You can only be prosecuted if you violate a law. Please cite the law that you would be violating.

Theft.

"ucla alum"

Yep.

UCL = University College London

Allow me my prejudices.

I think it's real simple, and never gets to a situation where the airline has to sue/bill the passenger or anything like that.

(1) FA notices you, and he/she will notice you.
(2) FA asks you to beat it back to Y. Maybe politely, maybe not.

So now you have a choice: go back to Y without another word, or give him/her some resistance, excuses, arguments, backtalk, and general pissiness.

Take the former route, and the situation is over and you enjoy the rest of your flight from your Y seat. Take the latter, and now you're a "security problem". You're disobeying crew commands and you're probably going to have a VIP greeting party waiting for you in the jetbridge when you land. Maybe it ends with just a lot of uncomfortableness and not actually a criminal charge...maybe not. Who really wants to find out?

I still can't believe people actually try this.

Here in New York we call it Theft of Services.

I suspect other states have similar laws.