I see no reason, why we shouldn´t do both things. Change old and new cards.
And there have been good nerfs of existing cards e.g. the nerf of the 13/4 mortal Phase Dragon to a 8/6 or 10/6 immortal one.
The change of Carpace Shield to Thorn Carpace has been, long time after Carpace Shield was released.
And I don´t think that this was a bad change.
All in all I think, our system works and that´s all what counts for me.