Originally Posted by
canadiancow
So it's a little more complicated. If the Y segment doesn't drop off the PNR when the upgrade is cleared, then in theory both seats could be valid and active.
That's rare (but I've seen it).
Almost 100% sure I saw it happen, too.
Earlier this year I sat beside a guy who boarded into PY. He had been upgraded to J but he didn't know it. The gate agent came up to him shortly after "boarding complete" was called, handed him a shiny new J boarding pass, and off he went.
But nobody came up from Y to take his seat. The flight went out "full", except for this one PY seat beside me.