Oh dear - I don't think some of the responses in this thread represent FT's finest hour.

I don't see any evidence of the OP demanding the swap, nor of the person who swapped seats caring one way or another about being on the UD or LD. What I guess happened was that the CSD noticed the OP's slightly emotional state and asked what was wrong, and probably thought that the first and simplest thing to do was to ask the person sitting next to her, rather than going downstairs and asking someone there (who might not have even been on board yet). Occam's razor (ish) - when you're busy, you do the simplest thing first!

I equally suspect that the only action taken by staff when the OP called up and at (very busy airport) JFK was to look at the seat map to see if there were any pairs free.

And in a normal situation I'd agree that that is all that anyone should expect - if someone is willing to move once onboard, then great, but don't expect it.

What I think is different in this scenario, as someone has already mentioned, is that given it was an a/c change, very many people in the cabin would have already been moved around. In that scenario, it is actually much more straightforward to find someone who has been moved but hasn't checked in yet, and simply swap their seats again. Since it wasn't a seat they'd selected in the first place, there's no harm done. But that does take a bit of time and willingness, which seems to have been absent on this occasion - as mentioned above, this is quite possibly due to the workload at JFK of the staff involved.

Something similar happened with me a couple of years ago coming back from ICN - I was "upgraded" at the gate from a WT+ window to a CW centre seat, and refused it. Rather than - say - further upgrading the person who'd been given my original seat and giving it back to me (which I suggested), they found another upgrade to CW in an aisle seat who hadn't yet got their new boarding card, and swapped the two of us around. I was only mildly disgruntled at the outcome, and they were none the wiser and happy with their upgrade.