Back when Tyranny of Dragons was new, and people were raging about how they’d changed the lore on the Cult of the Dragon, I did a deep dive into the older 2e and 3e books, and it turned out that the setup in Tyranny of Dragons was something that actually built off existing lore.
The 2e book,
Cult of the Dragon, establishes that Tiamat was getting her faithful to infiltrate and take over Cult of the Dragon cells. Here's my original post about that:
D&D 5E (2014) - Enhancing "Hoard of the Dragon Queen" (Practical stuff to try at your table!)
The 3.5e
Dragons of Faerûn book established that Tiamat was behind Sammaster's activation of the dragon rage mythal. He had hoped to use it to convince more living dragons to become dracoliches to avoid going insane, but I'm thinking Tiamat was playing 4D chess and was hoping that Sammaster would not only be defeated but the mythal would also be destroyed -- which is exactly what happened. I'm sure Tiamat would've viewed that mythal as a major roadblock in her quest to take over Faerûn. Here's my original post about that:
D&D 5E (2014) - Enhancing "Hoard of the Dragon Queen" (Practical stuff to try at your table!)
I believe the idea that Sammaster misinterpreted the prophecy as being about undead dragons instead of living ones was also established in 2e's
Cult of the Dragon book, so that wasn't a lore change in the Tyranny of Dragons adventure either.
Lastly, I’ll also point out that in
The Rise of Tiamat, there is some factional in-fighting between the new Tiamat cultists and the remnants of the old dracolich cultists. One of the latter, Naergoth Bladelord, appears as a wight at the Well of Dragons. He was originally detailed (in his living form) in 2e's
Cult of the Dragon book.
Conversely, this alliance between PDKs and amethyst dragons feels like it comes out of left field and isn’t a natural progression on what has come before. I’ll reserve judgment until I see the full context in the book, though.
EDIT: I found some of my original posts and have edited the above post accordingly.