I've been upgraded for operational reasons (overbooking in Y cabin) on a couple of occasions when holding Sapphire on Cdn (before it left oneworld). This leads me to believe they do use a priority based on your oneworld status, should their own elites not be in the back cabin. (It was offered to me at check-in.) If the back cabin is not overbooked, you are quite unlikely to be upgraded, but I've always found the seat next to me has been left empty.