My lot also have onboard upgrade available (in certain cases)
When a pax lobs up and mentions their sore back/dodgy leg/three kids and wants an upgrade, I just smile and ask will they be using points or credit card? Works every time...
Like Leezyjet, we once had a case where the person refused to go back. Did the same thing, let him sit there and had Mr Plod come on for a little chat.
I do recall in one case a guy sneaking into First for awhile before being noticed, it was on a ULR flight too... unfortunately for him he had a corporate account or some such so by the time we landed the company had organised to have the fare difference billed to his company account. Bet he never tried that one again, the cheeky git!! Didn't at all help that he was unpleasant as heck to crew on frequent occasions... if he had been nicer they probably would have just had the duty manager give him a talking to... because of his stinking attitude he ended up forking out to the tune of 5 figures!!
I have no problem with mum/dad swapping seats if one has a kid... they've paid for a seat, so long as they are using A seat. But the issue is when you have say, one parent with the kid down the back and the other up front (usually business trip where they've paid to take the spouse and kid) On one flight we noticed that when they swapped over mum had brought the child as well. No can do, child fares apply for J and F with my airline, and she indignantly said why not, there's empty seats....
Going by that logic, when I get to my hotel I can have the presidential suite because it's empty??
Don't care the reason, if you have paid for one cabin and go and plonk yourself in another, it's stealing plain and simple. I too would be well hacked off if I had paid and someone else got it for free (as in, was not entitled to or had not paid for that benefit)
Staff tickets are another story since many duty tickets by contract MUST be in a premium cabin. If a pax has bought a Y ticket and wants to upgrade then it's the same as if they showed up to buy that prem ticket after the cabin is full... in most airlines duty tickets are firm and are pretty much the same as commercial tickets. So no the pax is not 'entitled' to go above the staff on duty...