More or less exactly what it says on the tin: How does the released and upcoming Fantastic Beasts film series, written by JK Rowling, fit into the existing Harry Potter canon? Do they have the same level of authority as the books, the utter lack of authority of the movies, or somewhere in between?
While as the answers have already made clear that the Harry Potter universe has no official canon policy, it is interesting to note that Rowling has said on twitter that these movies are canon.
There were obviously some major changes from the HP books to the films... Are we to take everything in these films as canonical?
J.K. Rowling: Yes, because I'm writing them!
Of course, Rowling has failed to address why the parts of the movies which aren't from her script should be canon just because she wrote other parts of it.
There's no "official" canon in Potterverse the way LucasFilm had.
So there's no way to answer your question precisely.
Having said that, JKR will (at least according to my interpretation of her quotes) be the screenwriter for the films (unlike the original 8 Potter films) - see the bolded text for confirmation, all quotes from BBC interview/article:
"It all started when Warner Bros came to me with the suggestion of turning Fantastic Beasts and Where to Find Them into a film," said Rowling.
"I thought it was a fun idea, but the idea of seeing Newt Scamander, the supposed author of Fantastic Beasts, realised by another writer was difficult.
"Having lived for so long in my fictional universe, I feel very protective of it and I already knew a lot about Newt. As hard-core Harry Potter fans will know, I liked him so much that I even married his grandson, Rolf, to one of my favourite characters from the Harry Potter series, Luna Lovegood.
She went on: "As I considered Warners' proposal, an idea took shape that I couldn't dislodge. That is how I ended up pitching my own idea for a film to Warner Bros.
the realistic out of universe canonicity would definitely be WAY more than original Potter films (where JKR wasn't a screenwriter),
but personally I would rate it slightly less than the books' canonicity because JKR as a screenwriter is under the control/influence of the director/producer of the movies.
Of course, this could all change if JKR officially states some canon rules.
In addition, as far as in-universe continuity, Rowling said:
"Although it will be set in the worldwide community of witches and wizards where I was so happy for 17 years, Fantastic Beasts and Where to Find Them is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world.
Now that we have established that J.K. Rowling will indeed be writing the screenplay to Fantastic Beasts and Where to Find Them, I wanted to answer your actual question. According to JKR, Fantastic Beasts will not be an extension of the Harry Potter universe:
"Although it will be set in the worldwide community of witches and wizards where I was so happy for seventeen years, Fantastic Beasts and Where to Find Them is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world. The laws and customs of the hidden magical society will be familiar to anyone who has read the Harry Potter books or seen the films, but Newt's story will start in New York, seventy years before Harry's gets underway."
J.K. Rowling to Write Harry Potter Spinoff -- NPR -- 9.12.13
So, Fantastic Beasts will not fit into existing Harry Potter canon.
Since we haven't seen the movie yet, it's impossible to assess its authenticity in comparison to the Harry Potter movies, but I would suggest that perhaps Fantastic Beasts might have a higher level of authenticity within the wizarding world's canon (which stands alone from Harry and his story) because it's a stand alone set within the wizarding world. This is just an idea, though, on my part.
As a movie, compared to the Harry Potter movies, in the same way as I mentioned above, I think Fantastic Beasts would have greater authority. One, the author herself is writing the screenplay, and she is very protective of her universe; she's also very exacting. Two, while the Fantastic Beasts and Harry Potter movies come from the same universe, their stories do not intersect. While the book Fantastic Beasts and Where to Find Them is a bestiary, it also has a brief one page bio of Newt Scamander and an introduction that has a good bit of canon information on Magizoology. Whether JKR will use this in her screenplay, I don't know, but should she use her existing canon, and build Newt's story on that canon foundation, it would put Fantastic Beasts closer to JKR's personal wizarding canon as a movie.
According to this the films will be indeed written by JKR but I doubt they can be written according to the book strictly, as the book is not a story, but rather an documentary or journal of the magical creatures in the Harry Potter world. The movies can indeed be just about the trip of the author but it is likely more action will be needed to make a good movie.