I was definitely at First rather than Concorde but didn't feel like pressing the issue since I wasn't 100% sure of the rules and she did scan the boading pass (I presented both for the flights with the transatlantic flight in First on top, though she scanned the second) so I assumed it was the computer that made the decision rather than her. I mostly wanted wireless and a soda so either lounge suited my needs. I suppose I could try again in a few days when I take the same itinerary in the opposite direction but I had planned to use AA's lounge in T3.