>>>>But, instead of the GA handing me my new boarding pass, it was another pax!

Was the passenger who came aboard with your boarding pass the same coach passenger who was designated to commandeer your plumcake exit row seat? If so, then everything worked out to delightful perfection. That person got the nice coach seat, and you got the upgrade, in a pleasantly public way. If it was a random coach passenger (which somehow I doubt) then it would be deeply strange. But if it was the same person who was kicking you upstairs who would be taking your seat, then this would be kind of cute, and worth a laugh!