The same thing happened to me earlier this year on MSP-EWR, when I went from 1D to 12B (my original Y+ seat). The original pax showed up with a BP for my F seat, and I didn't know otherwise I'd been downgraded, and the F/A looked up my new (well, old) seat assignment in his little tablet and told me to head back to Y.
The GA very quickly came on before the door closed to let me know that a late connecting pax had showed up, but that was the end of the discussion. I chalked it up to the GAs jumping the gun in terms of processing upgrades, and she was super apologetic about it. It didn't really cross my mind to ask for comp, honestly.