arguments over what is "canon" (I find that a rather, ummm, loaded term 
) often boil down to whether a writer strays from the fandom's dominant OTP
I must admit that the term "canon" kinds of weirds me out; it always makes me think of the Council of Nicea deciding what books should go in the Bible.
But that's just a quirk.
And yeah, 'canon' is a loaded term with some disagreements about where it begins and ends. I've seen some stories referred to as 'not canon' even though the author didn't change anything from the original; with the reasoning that, according to the speaker's POV, some character (usually Ennis, sometimes Jack) has been changed enough that the character isn't the same.
"AU" can get into some pretty alternate universes, too. "Human Interest" and "Two Crows Joy" change the ending of the original but pretty much keep to Ennis and Jack as we know them in the story and movie. But others put Ennis and Jack on the
Titanic, in Paris, Ennis as a New York cop, Jack as a slutty truck stop whore, etc., etc.
That brings up the question as to whether these are the same characters at all, or original ones inspired by the
Brokeback principals. IMO, when stories of this type are well-written, it would sometimes be a good idea for the author to just change the names: under those circumstances the story could be published for profit without any copyright issues.
