It's an interesting question -- there's no logical reason why you couldn't do it, but in practice it's mostly going to be people annoyed at the new canon who do it. Being something of a canon stickler I probably wouldn't bother reading such a fic unless I had solid reason to suppose that it would be worthwhile. Even with fics written pre-HBP or pre-OotP, it can be disconcerting to read them for the first time now and suddenly have to mentally unpick a lot of what you normally assume about the Potterverse. It's a lot easier for fics I read for the first time pre-HBP, or sequels thereof, because that version had time to become familiar before new canon hit it.
(I've sometimes thought it would be an interesting and tricky challenge to throw out to write a reasonable length OotP-base or GoF-base or (for the truly ambitious) PoA-or-earlier-base story that used no information that was revealed later than the chosen book, but was nevertheless still compatible with the current story!)
Anyway, I'd call this sort of thing 'Alternate Reality', I think, and reserve 'Alternate Universe' for something where the change is more specific (e.g. 'DE!Peter induces Harry's birth early so MPP&L don't become targets'). But if people keep writing fanfic after book 7 (and why not, although there will probably be significantly less of it), a lot of it is going to be of this type, as time passes and details get blurred. It could well be the case that the characters are just used almost as archetypes, for their general roles and personalities, without any particular attempt to fit with more than the general idea of the story of the books, the way FFN poorer writers and hyperkeen slashers often do now anyway -- as with the many Holmes pastiches, films etc that ignore the details of what Conan Doyle wrote and have The Grear Detective fighting WW2 etc. (My vague idea for a crossover fic about the story of the giant rat of Sumatra, for which the world is not yet ready, just fell apart when I re-read the Holmes story it was mentioned in. Canon stickler, you see. :D)
no subject
(I've sometimes thought it would be an interesting and tricky challenge to throw out to write a reasonable length OotP-base or GoF-base or (for the truly ambitious) PoA-or-earlier-base story that used no information that was revealed later than the chosen book, but was nevertheless still compatible with the current story!)
Anyway, I'd call this sort of thing 'Alternate Reality', I think, and reserve 'Alternate Universe' for something where the change is more specific (e.g. 'DE!Peter induces Harry's birth early so MPP&L don't become targets'). But if people keep writing fanfic after book 7 (and why not, although there will probably be significantly less of it), a lot of it is going to be of this type, as time passes and details get blurred. It could well be the case that the characters are just used almost as archetypes, for their general roles and personalities, without any particular attempt to fit with more than the general idea of the story of the books, the way
FFNpoorer writersand hyperkeen slashersoften do now anyway -- as with the many Holmes pastiches, films etc that ignore the details of what Conan Doyle wrote and have The Grear Detective fighting WW2 etc. (My vague idea for a crossover fic about the story of the giant rat of Sumatra, for which the world is not yet ready, just fell apart when I re-read the Holmes story it was mentioned in. Canon stickler, you see. :D)