They definitely can do it, though no idea how easy it is. I did YYZ-MXP last year, Lat fare upgraded at booking, and on the morning of the flight I got the dreaded downgrade notification. Arrived at the airport to be pleasantly surprised that the downgrade was only on YYZ-YUL, and was given separate boarding passes for each leg. Ended up clearing back into J at the gate, but still had different seats for YYZ-YUL and YUL-MXP.