Last year, I used miles to upgrade a friend of mine travelling in W+Y (YUL-CDG-VIE) in advance (not OLCI upgrade); and only the TATL segment was upgraded that the itinerary becomes YUL-CDG in J and CDG-VIE in Y; he was able to get into the 2F lounge at CDG at transfer by scanning the CDG-VIE boarding pass.