[…] because the point of AUs is to explore canon by diverging from it. This part of their relation to the source defines them as AU. Only when the divergence — sometimes radical, sometimes small — is pointless and not related to an exploration of the source, the AU will fail to work as fanfic. Even though the link between the AU and the canon is still there, the reader can’t make sense of why this divergence was chosen, what the point of it all is. So the question of divergence is never really one of “how far” but always one of “how well” and “how deliberate”. If there are compelling reasons to change canon or even characterization in a certain way, the AU will be successful in exploring canon and characterization, and thus successful as fanfic, even if it is diverging widely. However if the changes are arbitrarily or pointless the AU will fail, even if the changes are small.
If you know me outside of Tumblr you hear me complain about all kinds of badfics a lot, and pointless AUs are often my main source of annoyance. This article (from 2001, so some of the information is hilariously outdated) presents the best explanation of what a good Alternate Universe story should be, I wish more fic writers gave the matter some thought.