I need something clarified. It seems they've retconned the Time of Legends Nagash/Vampires series. In them, Nagash created the immortals. After Nagash died, Neferata sought to recreate the immortal elixir, but screwed up and made vampires instead. She never fought alongside Nagash as one of his 'Mortarchs'. If she had, she would have been an immortal rather than a vampire, in fact vampires wouldn't exist at all.
What changed? Have the immortals been retconned and instead Nagash made the vampires? It seems odd to retcon Time of Legends as they're essentially the Fantasy equivalent of the Horus Heresy books.