I feel like people throw the word 'retcon' around when it's not applicable. Unless the new information directly conflicts with and explains away a previously established fact, I don't see it as a 'retcon'. I see it as adding in detail to the world.
While I agree that retcon is used far too liberally, in this case it's... well, what Patchwork said because it was firmly established from DAII that the Dalish trade extra mages around between clans. And if you fight the Dalish in DAO, I'm pretty sure they had more then three mages. I know Lanaya, Elora, and Zathrian were among them, but if Lanaya competed with others it implies the clan had more mages (though they could've been swapped around).
While the Dalish have many faults, I very much doubt any clan other then outright dumbasses would even think of doing something like this to a child. Casting her out into the wild with no survival training or magical training and only a few days of supplies.
Still, this is probably an isolated clan being dickish.
Not that I accept it, personally, in my universe. It exists purely to make the Dalish seem "bad" in the minds of players, because some people will take the actions of one clan and apply it wholesale.